cqlsh should automatically disable tracing when selecting from system_traces

patch by Philip Thompson and Mikhail Stepura; reviewed by Mikhail Stepura for CASSANDRA-7641
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	bin/cqlsh
Merge branch 'cassandra-2.1' into trunk
Merge branch 'cassandra-2.0' into cassandra-2.1.0
Merge branch 'cassandra-2.1.0' into cassandra-2.1
Merge branch 'cassandra-2.1' into trunk