fix(gs): Prevent duplicate minimap parts and use SinglePromiseInstance

[splixcord]

Prevent minimap parts appearing as duplicates or in the wrong place, this happens when the server or arena worker hangs long enough for more parts to be requested before the earlier one finished awaiting.
Fixes #130

[jespertheend]

I think it might still cause some issues this way because the arena.getMinimapPart() calls will start buffering.
Ideally the loop() doesn't make any new #updateNextMinimapPart() calls when one is already running, that should relieve the worker from some stress.

There's a couple of ways to go about this.

• Renda comes with a SingleInstancePromise class which is meant for exactly this. Its usage should be pretty straight forward, but you can take a look at this file for an example.
• You could set a boolean such as isUpdatingMapPart and then skip the call from inside the loop while it is set.
• You could call #updateNextMinimapPart() from within a while (true) loop and await a promise that resolves using a setTimeout() within the loop. The server only has a single Game instance, but if it ever receives functionality to create and destroy Game instances, this loop will have to be cleaned up correctly.

I think I'd prefer the first option, since it doesn't rely on any 'global' state (the boolean getting set) and its name makes it clear why it's being used.

With one of these approaches your current changes technically wouldn't be necessary, but I still think it'd be nice to keep them around because it seems more robust.

[jespertheend]

LGTM, thanks!