They can stay in the memory pool forever because consensus process will never
accept these transactions (and maybe even block consensus process at all).
We're paging these hashes, so we need a previous full page and a current one
plus some cache for various requests. Storing 1M of hashes is 32M of memory
and it grows quickly. It also seriously affects node startup time, most of
what it's doing is reading these hashes, the longer the chain the more time it
needs to do that.
Notice that this doesn't change the underlying DB scheme in any way.
If we only have genesis block (or <2000 headers) then we might as well use
generic logic below with zero targetHash because genesis block has zero
PrevHash (and its hash will naturally be the last on the chain going
backwards).
Let upper-layer APIs like actor.Send() return it as well. Server can return
"already exists" which is an error and yet at the same time a very special
one, in many cases it means we can proceed with waiting for the TX to settle.
Sometimes it can be hard to persist all changes at ones, the process
can take almost all RAM and a lot of time. Here's the example of reset
for mainnet from 2.4M to 1:
```
anna@kiwi:~/Documents/GitProjects/nspcc-dev/neo-go$ ./bin/neo-go db reset -m --height 1
2022-11-20T17:16:48.236+0300 INFO MaxBlockSize is not set or wrong, setting default value {"MaxBlockSize": 262144}
2022-11-20T17:16:48.236+0300 INFO MaxBlockSystemFee is not set or wrong, setting default value {"MaxBlockSystemFee": 900000000000}
2022-11-20T17:16:48.237+0300 INFO MaxTransactionsPerBlock is not set or wrong, using default value {"MaxTransactionsPerBlock": 512}
2022-11-20T17:16:48.237+0300 INFO MaxValidUntilBlockIncrement is not set or wrong, using default value {"MaxValidUntilBlockIncrement": 5760}
2022-11-20T17:16:48.240+0300 INFO restoring blockchain {"version": "0.2.6"}
2022-11-20T17:16:48.297+0300 INFO initialize state reset {"target height": 1}
2022-11-20T17:16:48.300+0300 INFO trying to reset blocks, transactions and AERs
2022-11-20T17:19:29.313+0300 INFO blocks, transactions ans AERs are reset {"took": "2m41.015126493s", "keys": 3958420}
...
```
To avoid OOM killer, split blocks reset into multiple stages. It increases
operation time due to intermediate DB persists, but makes things cleaner, the
result for almost the same DB height with the new approach:
```
anna@kiwi:~/Documents/GitProjects/nspcc-dev/neo-go$ ./bin/neo-go db reset -m --height 1
2022-11-20T17:39:42.023+0300 INFO MaxBlockSize is not set or wrong, setting default value {"MaxBlockSize": 262144}
2022-11-20T17:39:42.023+0300 INFO MaxBlockSystemFee is not set or wrong, setting default value {"MaxBlockSystemFee": 900000000000}
2022-11-20T17:39:42.023+0300 INFO MaxTransactionsPerBlock is not set or wrong, using default value {"MaxTransactionsPerBlock": 512}
2022-11-20T17:39:42.023+0300 INFO MaxValidUntilBlockIncrement is not set or wrong, using default value {"MaxValidUntilBlockIncrement": 5760}
2022-11-20T17:39:42.026+0300 INFO restoring blockchain {"version": "0.2.6"}
2022-11-20T17:39:42.071+0300 INFO initialize state reset {"target height": 1}
2022-11-20T17:39:42.073+0300 INFO trying to reset blocks, transactions and AERs
2022-11-20T17:40:11.735+0300 INFO intermediate batch of removed blocks, transactions and AERs is persisted {"batches persisted": 1, "took": "29.66363737s", "keys": 210973}
2022-11-20T17:40:33.574+0300 INFO intermediate batch of removed blocks, transactions and AERs is persisted {"batches persisted": 2, "took": "21.839208683s", "keys": 241203}
2022-11-20T17:41:29.325+0300 INFO intermediate batch of removed blocks, transactions and AERs is persisted {"batches persisted": 3, "took": "55.750698386s", "keys": 250593}
2022-11-20T17:42:12.532+0300 INFO intermediate batch of removed blocks, transactions and AERs is persisted {"batches persisted": 4, "took": "43.205892757s", "keys": 321896}
2022-11-20T17:43:07.978+0300 INFO intermediate batch of removed blocks, transactions and AERs is persisted {"batches persisted": 5, "took": "55.445398156s", "keys": 334822}
2022-11-20T17:43:35.603+0300 INFO intermediate batch of removed blocks, transactions and AERs is persisted {"batches persisted": 6, "took": "27.625292032s", "keys": 317131}
2022-11-20T17:43:51.747+0300 INFO intermediate batch of removed blocks, transactions and AERs is persisted {"batches persisted": 7, "took": "16.144359017s", "keys": 355832}
2022-11-20T17:44:05.176+0300 INFO intermediate batch of removed blocks, transactions and AERs is persisted {"batches persisted": 8, "took": "13.428733899s", "keys": 357690}
2022-11-20T17:44:32.895+0300 INFO intermediate batch of removed blocks, transactions and AERs is persisted {"batches persisted": 9, "took": "27.718548783s", "keys": 393356}
2022-11-20T17:44:51.814+0300 INFO intermediate batch of removed blocks, transactions and AERs is persisted {"batches persisted": 10, "took": "18.917954658s", "keys": 366492}
2022-11-20T17:45:07.208+0300 INFO intermediate batch of removed blocks, transactions and AERs is persisted {"batches persisted": 11, "took": "15.392642196s", "keys": 326030}
2022-11-20T17:45:18.776+0300 INFO intermediate batch of removed blocks, transactions and AERs is persisted {"batches persisted": 12, "took": "11.568255716s", "keys": 299884}
2022-11-20T17:45:25.862+0300 INFO last batch of removed blocks, transactions and AERs is persisted {"batches persisted": 13, "took": "7.086079594s", "keys": 190399}
2022-11-20T17:45:25.862+0300 INFO blocks, transactions ans AERs are reset {"took": "5m43.791214084s", "overall persisted keys": 3966301}
...
```
We need to keep the headers information consistent with header batches
and headers. This comit fixes the bug with failing blockchain
initialization on recovering from state reset interrupted after the
second stage (blocks/txs/AERs removal):
```
anna@kiwi:~/Documents/GitProjects/nspcc-dev/neo-go$ ./bin/neo-go db reset -t --height 83000
2022-11-20T16:28:29.437+0300 INFO MaxValidUntilBlockIncrement is not set or wrong, using default value {"MaxValidUntilBlockIncrement": 5760}
2022-11-20T16:28:29.440+0300 INFO restoring blockchain {"version": "0.2.6"}
failed to create Blockchain instance: could not initialize blockchain: could not get header 1898cd356a4a2688ed1c6c7ba1fd6ba7d516959d8add3f8dd26232474d4539bd: key not found
```
Don't use cache because it's not yet initialized. Also, perform
safety checks only if state reset wasn't yet started. These fixes
alloww to solve the following problem while recovering from
interrupted state reset:
```
anna@kiwi:~/Documents/GitProjects/nspcc-dev/neo-go$ ./bin/neo-go db reset -t --height 83000
2022-11-20T15:51:31.431+0300 INFO MaxValidUntilBlockIncrement is not set or wrong, using default value {"MaxValidUntilBlockIncrement": 5760}
2022-11-20T15:51:31.434+0300 INFO restoring blockchain {"version": "0.2.6"}
failed to create Blockchain instance: could not initialize blockchain: current block height is 0, can't reset state to height 83000
```
It's useful to reproduce different execution problems including executions
stopped because of GAS limit. Satoshi representation is deliberately used,
because that's what is usually found in logs.
