FCPPluginClientTracker: Implement thread to garbage-colllect disconnected clients from the TreeMap
- This implements the case where a intra-node-FCP client stops using a a FCPPluginClient object and it gets GCed, thus nulling the WeakReferences in the TreeMap at FCPPluginClientTracker.
- It does not implement the case where a FCPPluginClient object is referenced by a networked-FCP FCPConnmectionHandler and the plugin to which it is connected is unloaded. In the latter case, the WeakReferences in the FCPPluginClient will be nulled, and those are not yet monitored by the code of this commit. Thus, update the FIXMEs in FCPPluginClient to do this. (FCPServer can be removed from the FIXMes as the code which was planned to be in FCPServer will be all in its own class FCPPluginClientTracker)