process: run domain specific methods, if used

It's unecessary to check if domains are used on every nextTick. When
lib/domains.js is loaded, override the default functionality with domain
specific functionality. This will cause code to run quicker in both
cases.

Code is also optimized for run-time, not shutdown. If popping on some
more methods before shutdown, and won't cause any errors, then remove
the additional check.