Add retries and delay for initial lease db connection on Kea start

When Kea starts and can not connect to lease db, it makes 4 tries in total in rapid succession. (under 5 seconds) If we have a slowly starting db engine, Kea may exit prematurely.

Existing parameters like "max-reconnect-tries", "connect-timeout", and "reconnect-wait-time" work only when the db connection is LOST after the initial startup and connection check.