Allow empty IN relations in SELECT/UPDATE/DELETE statements

patch by Aleksey Yeschenko; reviewed by Sylvain Lebresne for
CASSANDRA-5626
Merge branch 'cassandra-1.2' into trunk

Conflicts:
	doc/cql3/CQL.textile
	src/java/org/apache/cassandra/cql3/QueryProcessor.java
	src/java/org/apache/cassandra/cql3/statements/ModificationStatement.java
	src/java/org/apache/cassandra/cql3/statements/SelectStatement.java
	src/java/org/apache/cassandra/cql3/statements/UpdateStatement.java