Database model for Kea High Availability

As of Kea 0.6.0, the HA status is fetched directly from the monitored Kea servers and displayed in the UI. We'd like to move to a different model in which the Stork server is fetching the HA status from the servers and stores them in the database. The UI can then fetch this information from the db along with some additional information not present in the response to the status-get command. Such information may include things like last failover event seen from the Stork server's perspective or anything else that the server is able to gather from the Kea servers over a period of time.

This ticket extends the Stork database to accommodate the HA specific information.