Allow staggered requests

[ahmadbky]

Sometimes, the ManiaPlanet client fails to send an HTTP request. This leads to a lot of runs that are not saved by players. The game-mode should cache failed requests somewhere, and try to send them next time in a different route, that allows to specify at which time it tried to send the request.

Concretely, for now, the API should provide 2 new routes:

• /staggered/player/finished
• /staggered/event/:handle/:edition/player/finished

Both would accept this JSON body:

{
  "req_tstp": 99999,
  "body": { /* same body as their non-staggered version */ }
}