Remove direct access to substores from Store()
Remove direct access to substores from Store()

Remove methods that fetch the underlying layer, image, and container
store objects from the public Store interface.  They're still accessible
via type assertions, but ideally that's never going to be necessary.

Signed-off-by: Nalin Dahyabhai <f562fcb2a0b898bf0be1afc867ea92325d8f825f@redhat.com>

rhatdan/storage,rhatdan/storage,containers/storage,rhatdan/storage,containers/storage,containers/storage