Previously, regardless of whether writing to persistence backend is
successful or not, the logic always cleared `self.staged`. This is
changed to only clear `self.staged` after successful write.
Additionally, the written changeset (if any) is returned, and
`PersistBackend::write_changes` will not be called if `self.staged` is
empty.
This is a more generic version of `keychain::persist::*` structures.
Additional changes:
* The `Append` trait has a new method `is_empty`.
* Introduce `Store` structure for `bdk_file_store`.