Sleep/resume overflow

[fengb]

When the timer doesn't fire for... days, the counter overflows and does bad stuff. This really only happens when the laptop / browser goes to sleep.

We should add a hook to disable/reenable the runloop upon sleep/wake.

[fengb]

There's also been a regression with background processing. setInterval doesn't seem to be as guaranteed to run as setTimeout (although neither is actually guaranteed). The recommendation is the use a WebWorker for the timer, which feels a little heavy but it can work.