Add missing table name to DROP INDEX responses

Patch and review by Tyler Hobbs and Aleksey Yeschenko for CASSANDRA-7539
Merge branch 'cassandra-2.1.0' into cassandra-2.1
memtable_cleanup_threshold and memtable_flush_writers defaults
patch by jbellis; reviewed by bes for CASSANDRA-7551
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1' into trunk