Use setImmediate instead of setTimeout for yields

It's faster and still lets IO and signals through.
Tweaks to StatusPoller

- Lower STATUS_INTERVAL_MS from 500 to 50. Note that we were effectively
  doing 150 or 100 anyway: the _pollFiber was polling every 100, and the
  nudged _throttledStatusPoll was actually throttling at the default of
  150, not 500, because the Throttled constructor was being invoked
  incorrectly (without the named 'interval' option).

- Use the same throttler for _pollFiber and nudge.
Nudge console for every compiled source file

This will refresh the status bar more often and make the build tool more
responsive to signals.

Now that yield uses setImmediate, this has minimal effect on
performance.