FCPConnectionHandler: Register created FCPPluginClient objects at the FCPServer
- To have the call server.registerPluginClient() be thread-safe, we need to be able to prevent threads from reading from pluginClientsByServerPluginName before registerPluginClient() is finished. (The reason is explained in a comment which is added by this commit). To allow that, this commit replaces the ConcurrentHashMap with a TreeMap + ReadWriteLock. This also has the bonus that TreeMaps will only use as much memory as their content is currently taking. The backing table of HashMaps in Java never shrinks, it only grows. (I had commited it to be a HashMap previously nevertheless because we are in a per-connection class so this would be local to a single client)
- Also make member variables private, no reason to have them visible