Cache stop should clear thread-local ExtendedRiverMarshaller or their instance caches

The fix for ISPN-2372 was incomplete. We now clear the references from the thread-local marshallers to the cache itself, so it can be garbage-collected, but we don't clear the marshallers or their instance caches. If the cache values' object graph is very large, the cached marshallers will use up a lot of memory that could be garbage-collected (assuming the cache was indeed restarted and only one cache instance is running now).