Add checkpoint, rollback to to bank (#1662)

add linked-list capability to accounts

change accounts from a linked list to a VecDeque

add checkpoint and rollback for lastids

add subscriber notifications for rollbacks

checkpoint transaction count, too

tests/multinode.rs

@@ -889,7 +889,7 @@ fn test_leader_to_validator_transition() {
         Arc::new(RwLock::new(LeaderScheduler::default())),
     );
 
-    assert_eq!(bank.get_tick_height(), bootstrap_height);
+    assert_eq!(bank.tick_height(), bootstrap_height);
 
     // Shut down
     ncp.close().unwrap();