Part of #1133
It will help us to use big.Int to store amount of NEP5 tokens. As far as
big.Int doesn't have constant size, we shouldn't use `NEP5TransferSize`
constant anymore.
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.
