cleaned up comments in transition lib. WIP medic/medic-webapp#714
reformated transitions lib, no logic changed.
removed all filters from design doc. WIP medic/medic-webapp#714
added backlog changes processor. WIP medic/medic-webapp#714

"backlog" processor uses changes feed descending request to get changes
from last_valid_seq to the beginning and runs transitions again on docs
that have errors or transitions have never run.

Also minor refactor, created applyTransitions as a module export so we
can call it from the backlog processor.