feat: Limit parallel deletes

[apricote]

TL;DR

Limit the parallel ongoing deletes.

Expected behavior

To avoid too much strain on the API and possible rate-limit exhaustions we should limit the number of parallel deletes (introduced in #761). This could be made configurable with a preference too.

Not sure what a good default limit would be. We could calculate something from the default API rate limit. In practice 5 is probably high enough to give some advantage to the user, while not doing too much in parallel.

[jooola]

Shouldn't this be handled by some client side throttling when hitting some rate limits? For example, read the rate limits value and sleep a bit if we are about to hit the limit?