Forbid re-adding dropped counter columns

patch by Aleksey Yeschenko; reviewed by Jonathan Ellis for
CASSANDRA-7831
Merge branch 'cassandra-2.0' into cassandra-2.1.0

Conflicts:
	CHANGES.txt
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1' into trunk