Fix performance regression from CASSANDRA-5614

patch by Sam Tunnicliffe; reviewed by Aleksey Yeschenko for
CASSANDRA-6949
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/db/AtomicSortedColumns.java
	test/unit/org/apache/cassandra/db/RangeTombstoneTest.java