Reconnecting

[pvutov]

Currently connecting to an ongoing multiplayer match doesn't work, it would be nice to fix that.

Hint: We should use the OnClientConnected callback to get the unit setup done for new clients. The ideal would be to do all post-spawning initialization through those, so we don't have to maintain two cases (initialization directly after spawning for normal clients and late initialization for reconnecting clients).

Disconnecting would be more trouble and isn't worth fixing right now.