process: separate nextTick domain logic

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.

@isaacs this has been on my mind since the initial ticker changes, and wanted to throw it out there and see if it stuck. Part of the purpose was to explicitly enforce the use of `_tickCallback` regardless of using domains and w/o depending on `nextTick` to catch if a domain was in place.

And the anal side wanted to simplify the old logic flow (left) to the new logic flow (right):
![old-new-nexttick](https://f.cloud.github.com/assets/289174/203572/2813437a-813d-11e2-9eb8-47bab5cdcac4.png)