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.
It's not network-tied any more, network is only needed to
sign/verify. Unfortunately we still have to keep network in consensus data
structures because of dbft library interface.
We make it explicit in the appropriate Block/Transaction structures, not via a
singleton as C# node does. I think this approach has a bit more potential and
allows better packages reuse for different purposes.
