Override ThreadFactory so that only worker threads are daemon
but the event-loop is not, so it can be appropriately ref-counted
to accomodate Node.js semantics.
Add a finish() to Nodyn, and track any child Nodyns (vm.js) to shut
down any Nodyns no longer in use when its parent shuts down.
Apply appropirate ref-counting to setTimeout/setInterval and cancelling
of them.