Don't expose internal classes over JMX

Patch by marcuse; reviewed by slebresne for CASSANDRA-7879
Merge branch 'cassandra-2.1.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	src/java/org/apache/cassandra/tools/NodeProbe.java