database: Make LoadBalancer not yield DB objects that hopelessly lost the connection
* This is useful if the first slave picked went down and others
can be used, especially in longer running scripts.
* A possible improvement to this would be to eventually allow
retries by removing the bad handles from the load balancer,
since isOpen() will never change from false. This would only
be useful for very very long running CLI scripts and is
probably an edge case for now.
Change-Id: Iecfc4004b4b2289907a4645b431de19198790d6c