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.
buffer: check logic simplification

Checks have been simplified and optimized for most-used cases.

Calling Buffer with another Buffer as the subject will now use the
SlowBuffer Copy method instead of the for loop.

No need to call for value coercion, just place the ternary inline.