PoW verify_block_signature causes side effects

[mvandeberg]

The PoW verify_block_signature override causes side effects both in the pow metadata object and block producer balances.

The call to verify_block_signature implies that it should not have side effects because of the check_block_signtature option to apply_block.

There are two solutions to the problem:

• Rearchitect our system calls to have a read-only verification and write system call. Then require verify_block_signature is read only and update the PoW contract to follow the new arcitecture.
• Remove check_block_signature flag and allow verify_block_signature to have side effects.

[sgerbino]

For now, we will remove verify_block_signature, verify_passive_data, and verify_transaction_signature flags from submit_block_request and all the related places within koinos-chain.

As well as rename the system call to indicate it does more than verification: verify_block_signature -> process_block_signature