feat: add timer support for legacy::Pool

[dswij]

Addresses hyperium/hyper#3167

Note that previously we're using tokio's Interval rather than Sleep. This PR tries to emulate the behavior for Pool using Sleep instead, so that:

1. No extra trait for Interval
2. User only needs to supply a Timer rather than a Timer and also an Interval.

[seanmonstar]

Woops. Adding this would technically be breaking.

I suppose we could just wrap it in the local Timer struct, and pass that? It will mean double Arc-dyn, which is unfortunate. Maybe we can find a clever way to indicate to hyper::rt::Timer that we don't need to double Arc it?

[dswij]

Whoops yeah you're right, forgot about this one.

For avoiding double Arc-dyn, we can probably come up with something using Deref and or TryFrom, though I feel it's a bit hacky. (something like, if it's from local Timer then don't need to wrap Arc)

I was playing around with unwrap_or_clone just to realize it's still experimental. 😅 https://doc.rust-lang.org/std/sync/struct.Arc.html#method.unwrap_or_clone