Avoid NPEs when receiving table changes for an unknown keyspace

patch by Robert Stupp; reviewed by Aleksey Yeschenko for CASSANDRA-7689
Merge branch 'cassandra-2.1' into trunk
Fix CHANGES.txt
Merge branch 'cassandra-2.0' into cassandra-2.1.0
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1' into trunk