Implement proper cleanup of FCP event-notifications client connections:
- Keep a ReferenceQueue of the WeakReference<PluginReplySender> so we can determine whether some of the WeakReferences got nulled
- Monitor the ReferenceQueue in a daemon-thread and remove entries from the client-lookup-table if some got nulled
- This IS rather bloated but I couldn't think of a better way of doing it :|