* Add service to track the most recent optimistically confirmed bank
* Plumb service into ClusterInfoVoteListener and ReplayStage
* Clean up test
* Use OptimisticallyConfirmedBank in RPC
* Remove superfluous notifications from RpcSubscriptions
* Use crossbeam to avoid mpsc recv_timeout panic
* Review comments
* Remove superfluous last_checked_slots, but pass in OptimisticallyConfirmedBank for complete correctness
(cherry picked from commit 89621adca7)
Co-authored-by: Tyera Eulberg <teulberg@gmail.com>