Reduce lock spam in XioPortal SubmitQueue.

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 scan finds no work.

xio: avoid starving run loop, don't stop loop in shutdown()
     Fix 2 issues flagged in review by Alex Rosenbaum.
     * In the XioPortal main loop, the recent reduce lock contention change
       also made easier to starve Accelio under steady send work.  Restore
       the original behavior.
     * Remove the call to xio_context_stop_loop() in XioPortal::shutdown(),
       to ensure that Accelio can finish cleaning up.

Move queue guard check.
Release der spinlock.

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