Fix validation when adding static columns

patch by slebresne; reviewed by iamaleksey for CASSANDRA-7730
Better error message when trying to add a collection with the same name than a previously dropped one
Merge branch 'cassandra-2.0' into cassandra-2.1.0

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