WIP src: don't check closing handles on beforeExit

uv_loop_alive() returns true if there are closing handles. This causes an issue running the beforeExit event and a timer has been unref'd:

process.on('beforeExit', function() {
  setTimeout(function() { }, 100).unref();
});

The event loop also shouldn't run one additional time or else the timer may execute an additional time.

This is a WIP commit, and mainly opened to raise discussion around the problem at hand.

R=@bnoordhuis