fix: Excessive thread use

[cwhite-rippling]

Step 1:

• Instead of spawning a thread, use tokio to sleep until tokio thinks we should wake up.
• Then when we wake up, check if the correct amount of time has passed.

Step 2:

• yes: Poll:Ready
• no: Repeat Step 1

Enjoys (most of) the performance of tokio without any of the code :). State free, initialization free!!

Also when taking a second look at this code I realized I can express this in a much simpler manner. Originally I just patched the thread portion of the future implementation, but by using tokio I don't actually need to implement a future at all. I turned ~50 lines of code into <10.

pub async fn sleep(duration: Duration) {
    let deadline = SuspendUnawareInstant::now() + duration;
    let mut now = SuspendUnawareInstant::now();
    while now < deadline {
        tokio::time::sleep(deadline - now).await;

        now = SuspendUnawareInstant::now();
    }
}

[brhoades]

Let's mention the bug fixed in the PR title (potentially excessive thread use) and describe it in the description.

[brhoades]

LGTM, but let's add a regression test.

[brhoades]

Overall, looks good.

Before we merge, can we add a regression test which would have blown up with the prior impl? I expect spawning and polling ~5k 500ms sleeps wouldn't have went well before.

[cwhite-rippling]

Great callout! Also just ran some tests and for 100k 500ms timeouts it took 15.57s using threads and 0.83s with tasks. Amazing speedup.