Don't exchange schema between nodes with different versions

patch by Aleksey Yeschenko; reviewed by Piotr Kołaczkowski for
CASSANDRA-6695
Merge branch 'cassandra-1.2' into cassandra-2.0

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