config states use hookenv.Config.changed, which can fail

The config states detect if a configuration item has changed using hookenv.config() and its changed method. This was designed assuming the only thing running in a hook context is a hook, which is now untrue. It is possible for an action to be invoked, which can check charm configuration using hookenv.config() and will flag changed config items as seen. If this action happens to be called between the config being changed and an actual hook being called (such as config-changed), then the configuration changed states will be lost.

The solution is for the base layer to cache the config itself in unitdata.kv(), so the cache only gets updated in charms.layer.basic.init_config_states()

Start at charms.layer.basic.init_config_states() and inspect how configuration changes are currently detected through hookenv.config(). Read the unitdata.kv() usage around the base layer, then verify that the cache is updated only during init_config_states() and that changes are not lost when an action runs before a configuration hook.

Written by the indexing model from the issue text.