TrackedOp: remove the init_from_message function

I'm not sure why we ever had this instead of just doing things in the
subclass constructor, and the semantics around it don't make much sense
for anything else (we called mark_event first anyway).

Signed-off-by: Greg Farnum <greg@inktank.com>
TrackedOp: introduce a _dump_op_descriptor function

Use this instead of direct access to the Message underneath when dumping
the TrackedOp.

Signed-off-by: Greg Farnum <greg@inktank.com>
TrackedOp: rename arrived_at to initiated_at, specify when constructed

Instead of relying on the message's get_recv_stamp, take a timestamp
when the TrackedOp is constructed. Rename get_received_at() -> get_initiated().

Signed-off-by: Greg Farnum <greg@inktank.com>
TrackedOp: introduce an _unregistered() function to let implementations clean up

Right now, the OpRequest uses it to clean up Message payload data.

Signed-off-by: Greg Farnum <greg@inktank.com>
TrackedOp: do not track a Message

Give it to the OpRequest (currently, the only TrackedOp implementation).

Signed-off-by: Greg Farnum <greg@inktank.com>
TrackedOp: do not require a Message when creating new Ops

Further parameterize the template to allow passing in an arbitrary parameter,
and move all the Message-based event marking from there into OpRequest.

Signed-off-by: Greg Farnum <greg@inktank.com>