bake: Abort "render first" jobs if we start using other pages. This prevents the baker from having one worker stuck on a very long job, like rendering the index page of a blog with lots and lots of posts.

cov-core==1.15.0
coverage==3.7.1
mock==1.0.1
pytest==2.7.0
pytest-cov==1.8.1
pytest-mock==0.4.3