Assign ServerConnection to Player before processing post-login packets

[wallenjos01]

Currently, when transitioning to a new server, the proxy begins auto-reading (and therefore processing) packets from the pending server connection before assigning that connection to the player via ConnectedPlayer::setConnectedServer. This causes a race condition if the backend server sends certain packets (particularly plugin messages) right after sending a JoinGamePacket. For example, the following has inconsistent behavior depending on when a plugin message is sent:

@Subscribe
public void onPluginMessage(PluginMessageEvent event) {
    if(event.getSource() instanceof ServerConnection sc) {

        // Will work consistently
        sc.sendPluginMessage(MinecraftChannelIdentifier.create("test", "message"), "Hello, World".getBytes()); 
        
        // Will often fail if the backend server sends a plugin message immediately after join.
        sc.getPlayer()
            .getCurrentServer()
            .orElseThrow()
            .sendPluginMessage(MinecraftChannelIdentifier.create("test", "message"), "Hello, World".getBytes());
    }
}

This PR reverses the order of two lines in TransitionSessionHandler, making the proxy wait to process packets sent by a backend server until after assigning the player's connection.

[Gerrygames]

Pretty sure that this does not make a difference. The code in question runs on the event loop of the server connection, so no packets from that connection can be handled during its execution.

[electronicboy]

Has this been tested? Because I'd imagine that the more bigger issue here is going to be that disabling auto-read doesn't promise to disable reading everything, and so there can still be stuff which ends up being processed, at least that is my understanding; The auto-reading re-enable + field update both occur within the same area.

[wallenjos01]

Has this been tested?

I created this PR because I ran into errors caused by this bug while working on a plugin for my private server, but just to be sure, I spent some time writing a plugin to test it. I tested with both Velocity build 385, downloaded from the website, and my own fork. I tested both Spigot 1.20.5 and Fabric 1.20.5 backends. I got about a 40% error rate when using build 385, and a 0% error rate using my fork. I have attached the relevant logs and code.

pmtest.zip