Fix the following linter warning:
```
pkg/rpc/client/wsclient.go:99:18 govet copylocks: literal copies lock value from *cl: github.com/nspcc-dev/neo-go/pkg/rpc/client.Client contains sync.RWMutex
```
1. Keep initDone check only for the places where cache is directly accessed.
We don't need to check it in other places, otherwise we have a mess of
duplicating checks.
2. Fix bug in code related to block deserialisation. There's no magic, so
checking that initialisation is done is not enough for proper block
deserialisation. We need to manually fill StateRootEnabled field.
3. Since transaction doesn't need network magic to compute its hash, we don't
need to perform Client initialisation before transaction-related requests.
4. Check that cache is initialised before accessing network magic.
5. Refactor the way Policy contract hash is fetched for Client requests.
We don't really need Client initialisation for that, it's OK to fetch Policy
hash on-the-fly.
When execution fails it's the only way to change the prompt showing the
last executed instruction. Example:
```
NEO-GO-VM > loadgo examples/engine/engine.go
READY: loaded 117 instructions
NEO-GO-VM 0 > run
Error: at instruction 3 (SYSCALL): failed to invoke syscall 805851437: syscall not found
NEO-GO-VM 8 > parse
Error: missing argument
NEO-GO-VM 8 > parse 123
Integer to Hex 7b
Integer to Base64 ew==
String to Hex 313233
String to Base64 MTIz
NEO-GO-VM 8 > unload
NEO-GO-VM > exit
```
We don't have a need to iterate over them at the moment, but since we're
changing the DB format in the next release anyway let's add this ability also,
just in case.
It couldn't be done previously with two maps and mixed storage, but now all of
the storage changes are located in a single map, so it's trivial to do exact
slice allocations and avoid string->[]byte conversions.
Private DAO is only used in a single thread which means we can safely reuse
key/data buffers most of the time and handle it all in DAO.
Doesn't affect any benchmarks.
Most of the time we don't need locking on the higher-level stores and we drop
them after Persist, so that's what private MemCachedStore is for.
It doesn't improve things in any noticeable way, some ~1% can be observed in
neo-bench under various loads and even less than that in chain processing. But
it seems to be a bit better anyway (less allocations, less locks).
They never return errors, so their interface should reflect that. This allows
to remove quite a lot of useless and never tested code.
Notice that Get still does return an error. It can be made not to do that, but
usually we need to differentiate between successful/unsuccessful accesses
anyway, so this doesn't help much.
