OSD::dequeue_op: pass op by ref

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

We'll want the op wq in the OSD to queue things other than OpRequests.
OpRequests and the new items will implement PGQueueableOp.

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD/ReplicatedPG: convert OSD op_wq to use PGQueueableOps

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