Add option to use TCP keepalives for client connections

A client can be disconnected without explicitly closing its TCP connection. This can happen if the client machine resets or there is a network disruption. In particular, load balancers can have an idle time that results in a connection becoming invalid. Impala can't really guarantee that the client will properly tear down its connection and the Impala side resources will be released.

TCP keepalive would allow Impala to detect dead clients and close the connection. It also can prevent a load balancer from seeing the connection as idle. This can be important for clients that hold connections in a pool.