fix QuiescentRegistryListener leak

`QuiescentRegistryListener` instances leak because they are never
canceled. `QuiescentRegistryListener` instances are essentially single
use, and should be cancaled after they detected monitorable changes.
The reason for this is because the embedded runnable calls
`DynamicMonitoringView.stop()` immediately followed by
`DynamicMonitoringView.start()`. This call to
`DynamicMonitoringView.start()` installs a new
`QuiescentRegistryListener` instance.

Author: pwinckles

parfait-core/src/main/java/io/pcp/parfait/QuiescentRegistryListener.java

@@ -48,6 +48,7 @@ public void run() {
                 synchronized (lock) {
                     if (lastTimeMonitorableAdded > 0 && clock.get().longValue() >= (lastTimeMonitorableAdded + quietPeriodInMillis)) {
                         LOG.info(String.format("New Monitorables detected after quiet period of %dms", quietPeriodInMillis));
+                        cancel();
                         runnable.run();
                         lastTimeMonitorableAdded = 0;
                     }