Add KeyCacheHitRate metric to CF metrics

patch by yukim; reviewed by jbellis for CASSANDRA-5868
Merge branch 'cassandra-1.2' into cassandra-2.0.0

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/io/sstable/SSTableReader.java
	src/java/org/apache/cassandra/metrics/ColumnFamilyMetrics.java
	src/java/org/apache/cassandra/service/CacheService.java