Limit number of parallel update requests

[asportnoy]

The Replugged API has a rate limit of 20 requests per second. However, the updater currently requests all updates at once (relevant code). The check should be throttled to 15 requests per second (give some wiggle room) and properly handle 429s.

In the future, we should add a route to the backend for a bulk update check to avoid needing a lot of requests like this.

[bop34]

I can take this.