sila_java.library.manager.ServerManager.close leaves the ServerManager Singelton in illegal state

In the method

	@Override
	public void close() {
		this.discovery.close();
		this.connections.values().forEach(Connection::close);
		this.connections.clear();
		this.servers.clear();
	}

the global ServerManager singelton sila_java.library.manager.ServerManager.instance should be set to null, too (with lock, of course). This way the next call of getInstance() (which should use double-checked-locking for better performance, btw) would create a new ServerManager instance. Without this change, using a closed ServerManager fails without any notice on discovery.