refactor(ext/http): use scopeguard defer to handle async drop (#20652)

Use the [scopeguard](https://docs.rs/scopeguard/) defer macro to run
cleanup code for `new_slab_future`.
This means it can be a single async function, avoiding the need to
create a struct and implement `PinnedDrop`

Async cleanup in Rust is awkward because async functions may be
cancelled at any await point when their Future is dropped.
The scopeguard approach comes from the following articles:
* [How to think about `async`/`await` in
Rust](http://cliffle.com/blog/async-inversion/)
* [Async Cancellation
I](https://blog.yoshuawuyts.com/async-cancellation-1/) (Reddit
[discussion](https://www.reddit.com/r/rust/comments/qrhg39/blog_post_async_cancellation/))

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