In Go structs must be copied when used as arguments.
To do so we must clone struct on VM level.
This is done by appending this struct to an intermediate array.
Process `init()` functions after global variables has been processed.
It's body is saved into the `_initialize` method where all
initialization is performed.
Closes#1192
1. We now have CMDGetBlockByIndex, so there's no need to request headers
first when we can just ask for blocks.
2. We don't ask for headers (i.e. we don't send CMDGetHeaders),
consequently, we shouldn't react on CMDHeaders.
3. But we still keep on reacting on CMDGetHeaders command as
there could be a node which needs headers.
It returned an error in case if block wasn't found (it might be when our
chain is lower). Fixed. It also should return all requested blocks, not
the first one.
In 121c9664b we should take into account isValid flag of
NativePolicy contract while retrieving MaxVerificationGas native
policy value. Otherwise we won't be able to get MaxVerificationGas
after the node was restarted, because this value is not truly
stored along with the other native policy values.
This commit fixes bug with headers verification after the node
restarting with an existing storage:
```
2020-08-03T12:52:56.158+0300 WARN failed processing headers {"error": "vm failed to execute the script with error: error encountered at instruction 0 (PUSHDATA1): gas limit is exceeded", "errorVerbose": "vm failed to execute the script with error: error encountered at instruction 0 (PUSHDATA1): gas limit is exceeded\ngithub.com/nspcc-dev/neo-go/pkg/core.(*Blockchain).verifyHashAgainstScript\n\t/home/neospcc/Documents/GitProjects/nspcc-dev/neo-go/pkg/core/blockchain.go:1454\ngithub.com/nspcc-dev/neo-go/pkg/core.(*Blockchain).verifyHeaderWitnesses\n\t/home/neospcc/Documents/GitProjects/nspcc-dev/neo-go/pkg/core/blockchain.go:1517\ngithub.com/nspcc-dev/neo-go/pkg/core.(*Blockchain).verifyHeader\n\t/home/neospcc/Documents/GitProjects/nspcc-dev/neo-go/pkg/core/blockchain.go:1175\ngithub.com/nspcc-dev/neo-go/pkg/core.(*Blockchain).addHeaders\n\t/home/neospcc/Documents/GitProjects/nspcc-dev/neo-go/pkg/core/blockchain.go:484\ngithub.com/nspcc-dev/neo-go/pkg/core.(*Blockchain).AddHeaders\n\t/home/neospcc/Documents/GitProjects/nspcc-dev/neo-go/pkg/core/blockchain.go:453\ngithub.com/nspcc-dev/neo-go/pkg/network.(*Server).handleHeadersCmd\n\t/home/neospcc/Documents/GitProjects/nspcc-dev/neo-go/pkg/network/server.go:454\nruntime.goexit\n\t/usr/local/go/src/runtime/asm_amd64.s:1373"}
```
