After contract is migrated there is no way to retrieve it's state.
This commit implements some metadata for NEP5 contracts, so that
values important for diplaying transfer log aren't lost.
This is an append-only log which is read only during some RPCs.
It is rather slow to get it from base every time we need to append to
it. This commit stores all NEP5Transfers in batches, so that
only a last batch needs to be unmarshaled during block processing.
