Add a Keep Alive operation to the Native Protocol

Socket level keep-alive option doesn't seem to bring a strong enough guarantee that the connection won't be dropped as some OS or Firewall level settings might force some different behavior.

To simplify things and bring the strong guarantee on this point that users expect, it would be interesting to add a NOOP or KEEPALIVE.

Note that we could also turn it into a HEARTBEAT operation that would expect a response from the server to allow client to regularly probe for Cassandra node health in situation where the queries traffic is too low to bring this probing implicitly.