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.