Kea configuration versioning in the backend
Kea configuration can be modified from Stork or externally. Stork should be able to detect that the configuration has changed (someone modified a configuration file) and assign a version number/id to the detected configuration. It should be possible to go back to one of the previous configurations and see what has changed.
This issue introduces suitable Stork backend changes to automatically store a trace of the old Kea configurations (perhaps in a separate table and perhaps using SQL triggers). Technically, configuration versioning is not required to implement other config-mgmnt tickets, e.g. subnet manipulation but it is related because subnet modification results in a Kea configuration change.