Fix case where pre-domain nextTick executing post-domain callbacks resulted in domains being ignored

Fixes #148

Does not seem to have a measurable impact on benchmarks:

https://docs.google.com/spreadsheet/ccc?key=0AnFpRHqZj2TQdExOeWpyVjBGd3IyRUM0cXhVYjMwTHc&usp=sharing