Fix and improve `Persist::commit` method

Fix and improve `Persist::commit` method

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.