Multiple calls to unitdata.kv().flush()

charms.layer.basic.clear_config_states() calls unitdata.kv().flush(), which will be the second time the storage is flushed in the hook. In the perverse case where a failure happens between these two calls, we can end up with invalid state.

I think this is a use case for an actual `@cleanup` phase in charms.reactive, per https://github.com/juju-solutions/charms.reactive/pull/20