Fix 2ndary index queries with DESC clustering order

patch by slebresne; reviewed by thobbs for CASSANDRA-6950
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/cql3/statements/SelectStatement.java
Fix merge
Merge branch 'cassandra-2.1' into trunk