Replace futures-timer with a hash-wheel timer

[yoshuawuyts]

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 (http/3 impl) has a relatively small implementation we may want to adopt (200 lines!). Though worth keeping an eye on mozilla/neqo#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 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!

[montekki]

@yoshuawuyts I will try to look into that