the backend can stash whatever values it wants in the config.
just like the DNS backends, we make verifyConfig return a sanitized config.
added benefit is that extra user fields (via API) are also not dumped into the db.
1. add setup, teardown hooks
2. move the managed mount setup and teardown to filesystem backend
3. remove this vague storage.js
we should convert storageApi into a real object, so we don't have to
keep passing apiConfig around
snapshot file tracks the snapshot directory. when app gets deleted,
the cleaner will remove the upstream snapshot directory when it runs.
cache files are used in rsync logic to track what was uploading into
snapshot in the previous run without needing to rescan upstream.
