os/bluestore: restructure deferred write queue

os/bluestore: restructure deferred write queue

First, eliminate the work queue--it's useless.  We are dispatching aio and
should not block.  And if a single thread isn't sufficient to do it, it
probably means we should be parallelizing kv_sync_thread too (which is our
only caller that matters).

Repurpose the old osr-list -> txc-list-per-osr queue structure to manage
the queuing.  For any given osr, dispatch one batch of aios at a time,
taking care to collapse any overwrites so that the latest write wins.

Signed-off-by: Sage Weil <sage@redhat.com>