update op added to a waiting queue or discarded

The decision to discard an op happens either in OSD or in PG.
The operation queue is now in OpWQ instead of op_queue.
The decision to add an op to a waiting queue regardless of its type is updated.
The decision to add a CEPH_MSG_OSD_OP to a waiting queue is described in full.

Signed-off-by: Loic Dachary <loic@dachary.org>