Fix OPENDJ-1348: Various connection pool implementations do not recover if the target server is powered off and restarted

* HeartBeatConnectionFactory: check for heartbeat timeouts when there is a bind/startTLS in progress. Use the pending bind/startTLS as a proxy for the heartbeat request
* CachedConnectionPool: flush all pending connection futures if an attempt to establish a new pooled connection fails. Improve statistics in toString() representation to make problems easier to diagnose during debugging
* AbstractLoadBalancingAlgorithm: minor change - notify event listeners after closing the monitoring connection so that statistics are more accurate.

Reviewed by Jean-Noel.