dynamic regions leak memory upon dispose due to ShrinkerTask

Creating a dynamic region via JCS.defineRegion and then destroying it via ComposeCacheManager.free does not fully reclaim all memory when the memory shrinker is enabled.

When shrinking is enabled, the org.apache.commons.jcs.engine.memory.shrinking.ShrinkerThread retains a pointer to CompositeCache which is never reclaimed.

A simple solution would be to have org.apache.commons.jcs.engine.control.CompositeCache.setScheduledExecutorService(ScheduledExecutorService) retain the ScheduledFuture as a field. When CompositeCache.disposed is called, we should call ScheduledFuture.cancel.