feat(kad): Limit in-flight Kademlia queries

[nazar-pc]

Description

Limit number of in-flight Kademlia queries to not exceed configured number of substreams

Notes

I went with a design where each query is first inserted into pending queue and QueryPool will maintain at most 32 queries active at any time, any queries that exceed the limit will remain in pending queue.

In order to guarantee insertion order of pending queries I have introduced an abstraction in second commit.

Since constant restricting inbound streams to 32 is now used for outbound too, I moved it up to the root of the crate and changed name to more generic.

Links to any relevant issues

#3235
#3236

Open Questions

Not entirely sure how to test higher-level behavior here.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[nazar-pc]

I just noticed that performance is severely limited because limit is enforced on Kademlia globally, while it needs to be on per-connection basis, changing to draft for now.

[dignifiedquire]

How does this deal with queries being canceled before they have been started? (this is a very relevant use case for my code, where I have run into limits before)

[nazar-pc]

It will be removed as if it was cancelled before the first .poll(), so there are no fundamental mechanics changes here.

PR as is does achieve the goal, but with poor performance due to global limit. I'll dig into making limits per-connection instead tomorrow and will update PR once I'm there.

[nazar-pc]

Replaced with #3287 that works on handler level instead