Avoid NPEs when receiving table changes for an unknown keyspace

patch by Aleksey Yeschenko; reviewed by Sylvain Lebresne for CASSANDRA-5631
Merge branch 'cassandra-1.2' into cassandra-2.0

Conflicts:
	src/java/org/apache/cassandra/service/MigrationManager.java
Merge branch 'cassandra-2.0' into cassandra-2.1
Merge branch 'cassandra-2.1' into trunk