process: streamlining tick callback logic

* Callbacks from spinner now calls its own function, separate from the
  tickCallback logic
* MakeCallback will call a domain specific function if a domain is
  detected
* _tickCallback assumes no domains, until nextTick receives a callback
  with a domain. After that _tickCallback is overridden with the domain
  specific implementation.
* _needTickCallback runs in startup() instead of nextTick (isaacs)
* Fix bug in _fatalException where exit would be called twice (isaacs)
* Process.domain has a default value of null
* Manually track nextTickQueue.length (will be useful later)
* Update tests to reflect internal api changes