Enable TCP keep-alive on backend connections

As discussed in https://datastax-oss.atlassian.net/browse/NODEJS-58, we
currently see hanging backend connections to Cassandra whenever a Cassandra
node is not cleanly shutting down.

Enabling TCP keep-alive should speed up error detection, but I'm not 100% that
such an error is actually propagating properly to users of the connection. The
connect callback is certainly not useful any more at that point, but
the hope is that destroying the socket will trigger another error on the next
activity.