DNM: new watch/notify framework

This is mostly complete.  The main remaining items are to convert rgw and
librbd over to use the new version of the API.

Questions:

 1) Not sure if the handle_failed_notify() notifications are actually useful.
    The original notifier doesn't know whether or not they were delivered, and
    if they really want to tell people they missed one notify they could send
    a second notify with that information.  The only information it conveys is
    that we were too slow acking a notify.

 2) Does the watch_check() API make sense?

 3) I wasn't quite motivated enough to pull all of the RadosClient watch/notify
    machinery into Objecter where it probably belongs.  As such, there is still
    an odd separation between the two that is awkward in a couple of places
    (getting error code back to librados, and the watch_check check).  Good
    enough?

 4) Do we need to optimize the OSD watch ping path at all?  Currently it cancels
    and reschedules a timeout event. 

 5) Is anything else needed for librbd to make good use of this?

The other thing I ran out of steam before doing is writing a reference user for
the API that uses all elements to full effect.  I think the use case I have in
mind is a shared cache with invalidates, like what RGW is doing.  But instead of
writing a toy or test implementation, it would be best to write a generic
shared-cache module that has all the features RGW needs and then migrate to using
it.  Maybe that will be simpler than fixing up RGW to make use of watch_check()
properly...?