Track metrics at a keyspace level

Patch by brandonwilliams, reviewed by yukim for CASSANDRA-6539
Merge branch 'cassandra-1.2' into cassandra-2.0
Track metrics at a keyspace level

Patch by brandonwilliams, reviewed by yukim for CASSANDRA-6539
Merge branch 'cassandra-1.2' into cassandra-2.0
Track metrics at a keyspace level

Patch by brandonwilliams, reviewed by yukim for CASSANDRA-6539
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	src/java/org/apache/cassandra/db/Keyspace.java
Merge branch 'cassandra-2.1' into trunk