Don't index tombstones.

patch by Mikhail Stepura; reviewed by Aleksey Yeschenko for CASSANDRA-7828
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-2.1.0

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