multiple contact points for MySQL and PostgreSQL

For the purpose of highly-available database connectivity, eliminating single point of failures in cluster nodes, and benefitting from Galera and Percona's active-active responsiveness, Kea could use the ability to specify multiple contact points in the same database access set.

Ideally, it would be less work if you could pass the responsibility of shuffling through the nodes onto the database library, like in Cassandra.

But if this is not an option, to avoid contention on selecting the connection to be used, a connection could be randomly chosen by each thread.

Design document: TODO