OSD::RemoveWQ::_process: use complete call back for final transaction

Otherwise, we might in _journaled_ahead hit the following sequence:
1) queue_op()
2) (apply thread): dequeue op, apply op
3) (finisher thread): run applied callback deleting sequencer
4) osr->dequeue_journal() <crash>

It's also more obviously necessary for parallel journaling.

Signed-off-by: Samuel Just <sam.just@inktank.com>