Stuck on oplog visibility when inserting directly into oplog namespace in standalone mode

Writing directly to the oplog namespace in standalone mode causes any readers of the oplog to be stuck waiting for oplog visibility.

SERVER-64125 changed the insert path to not set a timestamp in this case. That causes WiredTigerOplogManager::triggerOplogVisibilityUpdate not to be called because WiredTigerRecoveryUnit::_isTimestamped is false. Any reader waits forever for oplog visibility that never happens.