xio: avoid starving run loop, don't stop loop in shutdown() (REBASEME)

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.

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