Don't allow mixed multi/single column relations

Patch by Benjamin Lerer; reviewed by Tyler Hobbs for CASSANDRA-7711
Merge branch 'cassandra-2.0' into cassandra-2.1.0
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/cql3/statements/SelectStatement.java
	test/unit/org/apache/cassandra/cql3/MultiColumnRelationTest.java
Merge branch 'cassandra-2.1.0' into cassandra-2.1