Filter transactions from get_account_history result based on block number

[theoreticalbts]

Negative block number -> relative to head

[theoreticalbts]

Allowing negative block numbers for relative to head is a bad idea -- the client will likely want to track block numbers on its own.

We need a new index of (account ID, block number) -> operation_history_id where the ohid is the last op by the account in the block. The index should be disabled unless requested on the CLI. Disabling will increase performance but get_account_confirmed_history() will not be able to query based on block number.

[vikramrajkumar]

This issue was moved to bitshares/bitshares-core#61