multiserver: tracker subroute

[Berserker66]

What feature would you like to see?

Our Multiserver supports in theory websocket routing, currently we only use "/". We've seen multiple requests for real-time tracking of each player's checked locations and received items, as well as the chat flow. We could therefore add /tracker to the server, which would require some kind of sign-in token, possibly given to already authed players, which would allow attaching with a different network protocol that pushes all players location checks and item updates, as well as the chat.

An alternative method, though personally I think it would be messier, would be a new packet to "upgrade" an existing connection to go multi-slot.

I believe #3408 is one such request for this feature.