Use MessageChannel instead of setTimeout to avoid processing delays

[kkaefer]

Fixes #8671.

It works around the setTimeout caveats in IE (in my analysis, I found that setTimeout sometimes took more than 30 milliseconds just to execute the next call!) by switch to the universally supported MessageChannel objects. It's normally used to communicate between two different contexts, but you can also use it to "communicate" between the same context. It solves the performance regression reported in #8671 while maintaining the faster cancellation processing from #8633 by decoupling the messages sent from the counterpart from processing: it throttles the postMessage calls it receives from the sender to just one pending postMessage. This is essentially what setTimeout(..., 0) does too.

[mourner]

Now that the builds ran, looks like we need a MessageChannel fallback for Node.