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.
The order in which storage.Find items are returns depends on what items
were processed in previous transactions of the same block.
The easiest way to implement this sort of caching is to cache operations
with storage, flushing the only in `Persist()`.
