os/bluestore: reuse zombie OpSequencers by collection id

os/bluestore: reuse zombie OpSequencers by collection id

We can get a sequence that deletes and then recreates a collection where
the transaction removing the collection is delayed (due to pending IO on
its sequencer) but colleciton create is not (new sequencer).

Avoid any such reordering by recycling the old collection's sequencer if
the zombie_osr has not been reaped yet.

Fixes: http://tracker.ceph.com/issues/24550
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 199a110c9e2a42be64bd7fd873b6b172ce1347a8)