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.
Standards are NEP-11 and NEP-17, not NEP11, not NEP17, not anything
else. Variable/function names of course can use whatever fits, but documents
and comments should be consistent wrt this.
