Don't insert tombstones that hide indexed values into 2i

patch by Sam Tunnicliffe; reviewed by Aleksey Yeschenko for
CASSANDRA-7268
Merge branch 'cassandra-1.2' into cassandra-2.0

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/db/index/SecondaryIndexManager.java
Merge branch 'cassandra-2.0' into cassandra-2.1

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