Items were serialized several times if there were several successful
transactions in a block, prevent that by using State field as a bitfield (as
it almost was intended to) and adding one more bit. It also eliminates useless
duplicate MPT traversions.
Confirmed to not break storage changes up to 3.3M on testnet.
Reproduce behavior of the reference realization:
- if item was Put in cache after it was encountered during
Storage.Find, it must appear twice
- checking if item is in cache must be performed in real-time
during `Iterator.Next()`
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()`.
