Ceph » rbd

This work is essentially done, and has been since
October 2012 (or even earlier). However I held off
posting it for review because (as I recall) Josh thought
we should not put partial support in, we should wait until
it is more fully supported before adding it in.

At this point it sits behind a large set (40+) of
patches that have not yet been reviewed anyway.

Many of those patches are ground work to facilitate
some of the other code implementing the I/O path for
layered objects. But again, activating parent probe
support is waiting until things are more fully done.