Enable keepalive for native protocol

Patch by brandonwilliams reviewed by Sankalp Kohli for CASSANDRA-7380
Merge branch 'cassandra-1.2' into cassandra-2.0
Enabled keepalive for native protocol

Patch by brandonwilliams, reviewed by Sankalp Kohli for CASSANDRA-7380
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	conf/cassandra.yaml
	src/java/org/apache/cassandra/transport/Server.java