os/bluestore: handle zombie OpSequencers

os/bluestore: handle zombie OpSequencers

It's possible for the Sequencer to go away while the OpSequencer still has
txcs in flight.  We were handling the case where the osr was on the
deferred_queue, but it may be off the deferred_queue but waiting for the
commit to happen, and we still need to wait for that.

Fix this by introducing a 'zombie' state for the osr, in which we keep the
osr in the osr_set.

Clean up the OpSequencer methods and a few other method names.

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