introduce cumulative state_hash and expose it in API

[dshulyak]

in the vm state_hash is a hash of accounts that were changed in the current layer, we want to have a more robust approach to check that nodes in the network see the state the same way.

• compute and save cumulative_hash_current = hash(prev_cumulative_state_hash, state_hash) where prev_cumulative_state_hash for genesis is the state hash of genesis accounts
• expose it over API

TODO i will post suggested api a bit later