Fix CC#collectTimeOrderedData() tombstone optimisations

patch by Aleksey Yeschenko; reviewed by Jonathan Ellis for
CASSANDRA-7394
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/db/CollationController.java
Merge branch 'cassandra-2.1' into trunk