Microoptimize CTL I/O queues.

Switch OOA queue from TAILQ to LIST and change its direction, so that
we traverse it forward, not backward. There is only one place where
we really need other direction, and it is not critical.

Use STAILQ_REMOVE_HEAD() instead of STAILQ_REMOVE() in backends.

Replace few impossible conditions with assertions.

MFC after: 1 month

(cherry picked from commit 05d882b780f5be2da6f3d3bfef9160aacc4888d6)