Prioritize threads affected by user typing
To this end I’ve introduced a new custom thread pool type which can spawn threads using each QoS class. This way we can run latency-sensitive requests under one QoS class and everything else under another QoS class. The implementation is very similar to that of the `threadpool` crate (which is currently used by rust-analyzer) but with unused functionality stripped out.
I’ll have to rebase on master once #14859 is merged but I think everything else is alright :D
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