Fix potential AssertionError with 2ndary indexes

patch by slebresne; reviewed by beobal for CASSANDRA-6612
Merge branch 'cassandra-2.0' into cassandra-2.1.0

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