Auto merge of #7450 - benschulz:constellation-timer, r=jdm

Ordering guarantees for timers

This is an rough solution to the issue described in #3396. XHRs still do their own thing and an overall clean up is in order. Before I do that, though, I'd really like someone to sign off on the overall idea.

There's one major difference to what jdm layed out #3396: The timers remain with the window/worker and only the earliest expiring one is coordinated with the dedicated timer thread.
That means both the timer thread and the window/worker have to keep track of which timer expires next, which feels a bit wonky. However, the upshot is that there's no need for communication with the timer thread when a pipeline is frozen, thawed or dropped.

Most relvant parts are
 - the [`TimerScheduler`](https://github.com/servo/servo/commit/6f5f6619586379982321b5de7583d387df184504#diff-74137a6f50ab38e7a1e4d16920a66ce7R73), which is the new per-constellation timer task and
 - the [`ActiveTimers`](https://github.com/servo/servo/commit/6f5f6619586379982321b5de7583d387df184504#diff-86707d952414a2860b78bcf6c1db8e2eR34) which is what's left on the window/worker side.

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