Login event should be called after disconnect event, when we have multiple connections for the same player

Currently (Version: 1.16.4 / #393) when a player connects to the proxy, when another connection for the same player is still online (out of whichever reasons), it can happen that the login event for the new connection is called before the old connection has their disconnect.

This situation can lead to some problems for custom plugins / development with the order of events and you can't make sure that the order is always disconnect and than login. So you maybe have to do some manual checks in the login event.

As @electronicboy stated in Discord it "would need look into making it happen as a callback in netty if it lets us". It would therefore be good if this is checked so that you can always maintain a defined order of events for a player. Alternatively, one could generally think about whether a new connection should be allowed at all as long as an old connection still exists.

Trace the connection lifecycle and login/disconnect event dispatch, starting with the Netty callback mentioned in the issue. Determine how duplicate connections are handled and verify the change by reproducing the case; done means the old connection's disconnect event always occurs before the new connection's login event.