JmxProxy class responsible for communication between Reaper and Cassandra

JmxProxy can issue commands to Cassandra, as well as it subscribes to
events generated by StorageService mbean, from which it filters only
the repair-specific ones. Handling of repair events is done via the
RepairStatusHanlder interface implementation of which must be
provided when connecting the JmxProxy.

Change-Id: I607557b4f1d9c4dd6133d5622d42610c5071b5f6