In general, NEP5 contracts are not limited to int64. And we have an example
of pnWETH Flamingo token now (with 18 decimals) that easily overflows int64,
so for correctness we need to store big.Int.
And as TransferLog is shared for different purposes I've decided to not make
it variable-length on Neo 2.
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.
