Add per-CF range read request latency metrics

patch by Sam Tunnicliffe; reviewed by Aleksey Yeschenko for
CASSANDRA-7338
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/db/ColumnFamilyStore.java
	src/java/org/apache/cassandra/metrics/ColumnFamilyMetrics.java