directConnectAddress should be reset in DarkReflectiveMirrorTransport when switching server

[peterparnes]

If you have several servers and allow the user to switch between different servers then the directConnect-handling is a bit broken. When the client joins server one, the directConnectAddress is saved in DarkReflectiveMirrorTransport and when the client later joins a second server then the old directConnectAddress is used for connecting instead of waiting for an answer from the relay-server.

A simple solution is to add the following line at the top of "ClientConnect(string address)": directConnectAddress = "";

I didn't do a Pull request for this as I am not sure the solution is the best one.