It's cleaner to only load domain ticker logic when the domains are being
used. This makes execution slightly quicker in both cases, and simpler
from the spinner since there is no need to check if the latest callback
requires use of domains.
Not necessary, since we can handle the error properly on the first tick
now, even if there are event listeners, etc.
Additionally, this removes the unnecessary "_needTickCallback" from
startup, since Module.loadMain() will kick off a nextTick callback right
after it runs the main module.
Fix#4856
* 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
Trevor Norris
isaacs