Since _tickCallback and _tickDomainCallback were both called from
MakeCallback, it was possible for a callback to be called that required
a domain directly to _tickCallback.
The fix was to implement process.usingDomains(). This will set all
applicable functions to their domain counterparts, and set a flag in cc
to let MakeCallback know domain callbacks always need to be checked.
Added test in own file. It's important that the test remains isolated.
Trevor Norris