Fix issues w/ CONTAINS (KEY) queries on 2ary indexes

Patch by Benjamin Lerer; reviewed by Tyler Hobbs for CASSANDRA-8147
Consolidate Relation.Type and IndexExpression.Operator

Patch by Benjamin Lerer; reviewed by Tyler Hobbs for CASSANDRA-8147
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	src/java/org/apache/cassandra/cql/QueryProcessor.java
	src/java/org/apache/cassandra/cql3/statements/SelectStatement.java
	test/unit/org/apache/cassandra/db/CleanupTest.java
	test/unit/org/apache/cassandra/db/ColumnFamilyStoreTest.java
	test/unit/org/apache/cassandra/streaming/StreamingTransferTest.java