Rpc -> proper optimistic confirmation (#12514)

* 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

runtime/src/commitment.rs

@@ -104,8 +104,8 @@ impl BlockCommitmentCache {
     }
 
     pub fn highest_gossip_confirmed_slot(&self) -> Slot {
-        // TODO: see solana_core::RpcSubscriptions:
-        //self.last_checked_slots.get(&CommitmentLevel::SingleGossip).unwrap_or(&0)
+        // TODO: combine bank caches
+        // Currently, this information is provided by OptimisticallyConfirmedBank::bank.slot()
         self.highest_confirmed_slot()
     }