patch by Berenguer Blasi; reviewed by Benjamin Lerer for CASSANDRA-15842

The tests were constantly failing for the cdc test run due to the fact
that CDCSizeTracker.shutdown was throwing a NPE if the method was called
while CDCSizeTracker was not started.
The fix ensure that the test start the commitlog and that
CDCSizeTracker.shutdown is a NOOP if the tracker has not been started.