[FEATURE]: Fork aware mempool

[sgerbino]

Is there an existing issue for this?

• I have searched the existing issues

New feature

As a Koinos user, I would like a fork aware mempool such that transactions do not get forgotten on forks.

• Enhance statedb so that root can be backed by a std::map koinos-state-db-cpp#9
• Integrate statedb and replicate the workflow in account_history #81
• Modify add_pending_transaction to add transactions to each fork head #82
• When receiving new block broadcasts, we only remove the pending transaction from that particular fork #83
• Update RPC so that we have an optional block ID so that we can query the mempool on a particular fork -- default to the known head block koinos-proto#192
• Listen to irreversible broadcasts and move mempool irreversibility up #85

Note: Upon startup, the statedb root should be genesis (0x00). If transactions come in prior to a LIB, apply the transactions directly to the genesis node. When LIB comes in, attempt to attach it to previous, if there is no previous attempt to attach the node to genesis. If there is no genesis, report an error because something went wrong.

Anything else?

Right now it is possible for a transaction to end up on a fork, the cluster makes no attempt to re-apply this transaction on the main fork. This will cause bad UX for both users.