Improve PerRowSecondaryIndex performance

patch by Sergio Bossa; reviewed by Aleksey Yeschenko for CASSANDRA-6876
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/db/Keyspace.java
	src/java/org/apache/cassandra/db/index/SecondaryIndexManager.java