ability to detect Kea config changes (config-hash-get)

There was a discussion in Porto about detecting out of bounds configuration changes in Kea. The overall idea is that Stork should be able to detect somewhat easily if Kea's config has changed, e.g. by sysadmin or some external tool.

Couple ideas were discussed:

• storing timestamp of last modification
• using hash
• using monotonic counter
• using journal file or auditlog

The overall idea is that Stork (and other monitoring tools) should be able to reasonably easily answer the question whether configuration was modified or not. It is essential the question/answer should be relatively low cost as Stork and other monitoring tools tend to look at Kea's config frequently (e.g. every 15 seconds) and the config changes are typically rare events.

This requires a short design.