It no longer depends on blockchain state and there can't ever be an error, in
fact we can always iterate over signers, so copying these hashes doesn't make
much sense at all as well as sorting arrays in verifyTxWitnesses (witnesses
order must match signers order).
It's not needed any more with Go 1.13 as we have wrapping/unwrapping in base
packages. All errors.Wrap calls are replaced with fmt.Errorf, some strings are
improved along the way.
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"}
```
Disallow costly verification methods. We put this limit in policy
contract as it may be a subject to change in future.
In fact this value also overrides gas limit for header verification.
Close#1202.
