param socket_keepalive to set keep-alive on socket

Provide an option to enable [`setKeepAlive`](http://nodejs.org/api/net.html#net_socket_setkeepalive_enable_initialdelay) on the node TCP Socket for long running connections. This is a better option than pinging manually as mentioned in https://github.com/mranney/node_redis/issues/530 and https://github.com/mranney/node_redis/issues/247