Volume cloning - design sketch for 739

Design sketch for #739; builds on #771... only relevant parts are in https://github.com/ClusterHQ/flocker/commit/ca2a73355204e6d9371fc6470e8a3c925b048434
We don't have volume destruction yet.  If we did, I would want a test verifying that destroying a snapshot that has any clones fails and I'd want the memory implementation to pass that test.