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 (OOM or kill -9).

Enabling TCP keep-alive should speed up the detection of such hanging
connections somewhat, although with the default linux settings it should still
take 10s delay (default in this patch) plus 9x 75s keepalive interval (Linux
default settings, see
http://tldp.org/HOWTO/TCP-Keepalive-HOWTO/usingkeepalive.html).