Fix dropping collection column when it's the last regular column

patch by slebresne; reviewed by iamaleksey for CASSANDRA-7744
Merge branch 'cassandra-2.0' into cassandra-2.1.0

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/config/CFMetaData.java
	src/java/org/apache/cassandra/cql3/statements/CreateTableStatement.java