Fix merging schemas with re-dropped keyspaces

patch by Aleksey Yeschenko; reviewed by Pavel Yaskevich for
CASSANDRA-7256
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/db/DefsTables.java
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	src/java/org/apache/cassandra/db/DefsTables.java