Reportstate volume size floc 976

Fixes FLOC-976
I've got a feeling that it'd be better to continue using LoopingCall. 

That'll keep the `compare_volumes_by_uuid` free of unrelated scheduling code.
It'll also cut out the recursion in that function, which looks like it may result in a very long deferred callback chain if the volume doesn't become available. I'm not sure if long callback chains are still a problem, but they were once http://twistedmatrix.com/trac/ticket/411 and I'd be inclined to avoid this if possible by using high level APIs like LoopingCall.

See what @exarkun or @itamarst think.