Implement max_concurrency support

[GnomedDev]

Implements support for the max concurrency system. This allows bots on very large sharding to start up multiple shards in parallel to cut startup times down quite a bit. I've personally tested this using my bot with 256 shards and a max_concurrency of 16 and it worked flawlessly.

[mkrasnitski]

Might be better to separate concerns to make sure the rework is reviewed separately.

[GnomedDev]

Done, and drafted until that PR is merged.

[mkrasnitski]

This was discussed in the discord, but basically: When creating a batch of shards, to ensure both maximum batch size and avoid being ratelimited, their ratelimit keys should all be unique (calculated via shard_id % max_concurrency). That means that if a shard fails to start, when including it in the next batch for retry, it should replace a shard with the same rate limit key, in order to make sure the batch doesn't include any shards which are ratelimited together.

[GnomedDev]

@mkrasnitski is going to take over this effort.