Replace futures-timer with a hash-wheel timer

[`futures-timer`](https://github.com/rustasync/futures-timer/) is quite naive in its implementation, and we can probably do better. Something we may want to consider is building a hash-wheel timer impl to replace the existing timer.

[mozilla/neqo#127](https://github.com/mozilla/neqo/pull/127/files#diff-70ad500d4cb28767e4c01ebb6999b075) (`http/3` impl) has a relatively small implementation we may want to adopt (200 lines!). Though worth keeping an eye on https://github.com/mozilla/neqo/issues/190, where they're investigating possible other solutions.

Either way I think it's probably worth spinning up our own crate for this, and moving away from `futures-timer`. I've also gone and reserved the [`hash-wheel-timer`](https://crates.io/crates/hash-wheel-timer) name so we publish after we've built it.

Also if anyone's interested in taking this on feel free to comment below. This is not a high priority, but generally could make for some nice improvements. Thanks!