Add better database failover mechanisms

[rjl]

For high-availability installations it's vital to be able to detect that a database server is down and switch to a backup database immediately. The amavisd-new code supports database failover in a limited sense, but Maia needs to implement something at least as effective.

One issue to resolve is whether to revert to a primary server after switching to a backup server. In that instance, the primary server that just came back online may not have as current a data set as the backup server (until replication restores what the primary missed), so it may be better to continue with the backup server until some signal is received to force the switch back to the primary.