Remove futures-intrusive when using tokio

[paolobarbolini]

Currently implemented just for tokio and without waiting for all permits to be released on close

[abonander]

There unfortunately doesn't seem to be a good alternative for async-std. async-lock doesn't have the API we need, although they're apparently amenable to adding it if you have time to open a PR.

I've already added event-listener to fix #1764 so just using async-lock for all runtimes, assuming it gains the method we need, seems like the easier solution.

[paolobarbolini]

I've just tested tokio-console and being able to see what the various async resources are doing, including semaphores, is so nice that I'd want to have a different underlying implementation be used depending on the chosen runtime.

[paolobarbolini]

Would keeping futures-intrusive with async-std sound acceptable? I've implemented that.

[abonander]

That would be acceptable. What's with the cancelled check though?

[abonander]

Oh, it appears to be hung up, 4h 50m and ticking: https://github.com/launchbadge/sqlx/runs/7257076717?check_suite_focus=true

[paolobarbolini]

I'll look into it

[abonander]

Since it seems like you haven't had the time to come back to this, I'm going to close. Don't worry though, I'm currently doing work involving the runtime features and part of that is using the native runtime types where applicable.