Static columns with IF NOT EXISTS don't always work as expected

patch by slebresne; reviewed by iamaleksey for CASSANDRA-6783
Merge branch 'cassandra-2.0' into cassandra-2.1

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