ceph_test_objectstore: queue split on parent pg sequencer

ceph_test_objectstore: queue split on parent pg sequencer

This matches the OSD.

It also ensure that we drain transactions that precede the split, which
will include any finish_write calls that might otherwise attach the
Buffer to the wrong cache shard: _txc_finish() calls finish_write and
passes the cache shard without the cache shard lock held, but may block
waiting for split_cache() which then changes the destination collection's
shard.  Once it gets the lock and proceeds it would operate on the wrong
cache shard, leading to a failed assert later when the sharedblob is
trimmed.

Fixes: http://tracker.ceph.com/issues/24439
Signed-off-by: Sage Weil <sage@redhat.com>