Support DISTINCT for static columns

This also fix the behaviour when DISTINCT is not used.

patch by slebresne; reviewed by thobbs for CASSANDRA-7305
Merge branch 'cassandra-2.0' into cassandra-2.1

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

Conflicts:
	CHANGES.txt