OpTracker: give TrackedOp a default dump() function

Signed-off-by: Greg Farnum <greg@inktank.com>
TrackedOp: template OpTracker on only T[::Ref] (ie, OpRequest[::Ref])

Signed-off-by: Greg Farnum <greg@inktank.com>
TrackedOp: just make CephContext member public

This is the only reason we were making OpRequest a friend class, which
is just a bad idea.

Signed-off-by: Greg Farnum <greg@inktank.com>
TrackedOp: rework dump interface a little

Put only the bare essentials in the TrackedOp::dump() implementation, but
have that call into a _dump() function that implementations can specify.

Signed-off-by: Greg Farnum <greg@inktank.com>
TrackedOp: give people an _event_marked() notifier instead of a virtual mark_event()

This is less prone to getting broken.

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