Don't send schema change msg for no-op DDL stmts

Patch by Tyler Hobbs; review by Mikhail Stepura for CASSANDRA-7600
Merge branch 'cassandra-2.0' into cassandra-2.1.0
Merge branch 'cassandra-2.0' into cassandra-2.1
Merge branch 'cassandra-2.1.0' into cassandra-2.1