process: remove spinner

Remove the need to call start/stop the uv_idle spinner between
MakeCallbacks. The one place where the tick processor needs to be kicked
is where a user catches uncaughtException. For that we'll now use
setImmediate, which accomplishes the same task.
src: remove old comment code

Now that maxTickDepth no longer exists there's no depth index on
infoBox. Forgot to remove the comment about this.
process: use Tock for nextTickQueue items

v8 plays very well with constructed objects, so we're using one in the
nextTickQueue.