database: Make LoadBalancer not yield DB objects that hopelessly lost the connection

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