Handle deferred.

Signed-off-by: Matt Benjamin <matt@cohortfs.com>
Reduce lock spam in XioPortal SubmitQueue.

In SubmitQueue::deq, scan up to nlanes, exit immediately with
found work, recalling last lane.

Yield to Accelio iff a full can finds no work.

Signed-off-by: Matt Benjamin <matt@cohortfs.com>