Fix disconnected session activity (#675)

When a session is disconnected, the apiContext long-polling for messages
continues until resolved/rejected. This means that even after a session
is disconnected, a message can be received and handled.

This leads to bad behavior, as the session was already cleaned up and
the message cannot be handled correctly. The instance map was cleaned up
upon disconnect, so it will warn about unapplied changes to unknown
instances. (Like #512)

It's very easy to repro:
Connect a session, disconnect it, then save a change.


https://github.com/rojo-rbx/rojo/assets/40185666/846a7269-7043-4727-9f9c-b3ac55a18a3a

-----------

This PR fixes that neatly by tracking all active requests in a map, and
cancelling their promises when we disconnect.

plugin/src/ServeSession.lua

@@ -285,6 +285,8 @@ end
 function ServeSession:__mainSyncLoop()
 	return self.__apiContext:retrieveMessages()
 		:andThen(function(messages)
+			Log.trace("Serve session {} retrieved {} messages", tostring(self), #messages)
+
 			for _, message in ipairs(messages) do
 				local unappliedPatch = self.__reconciler:applyPatch(message)