Improve cqlsh waiting for a trace to complete

Patch by Tyler Hobbs; review by Aleksey Yeschenko for CASSANDRA-7626
Fix tracing of concurrent range slices

Patch by Tyler Hobbs; review by Aleksey Yeschenko for CASSANDRA-7626
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1' into trunk