snapshot dependencies: move info into their own files

The file contents are protected by container_disk_lock()
and at lxc_clone by the new container not yet being
accessible.

The problem with using $lxcpath/$name/config is that
api users could end up calling c->save_config() with
a cached old value of snapshots/rdepends.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>