Mark connection as unhealthy when disconnected

This commit fix the following problem :
- use a haproxy between node-cassandra-cql and cassandra
- set timeout in haproxy
- wait lot of time without activity. Haproxy will kill unused connection.
- send some cql requests
=> All requests will fail

This is probably not the best way to fix the problem, but it's works. Let me known if there is a better solution.