Quickcache

Wrote an easy "all-purpose" cache decorator and used it in the following places

- to make `get_questions` only do work when the xform source changes
- fetching build config doc
- fetching those one-off config docs (indicators, domain module map, etc.) instead of cache_core

Hoping this will make the activation energy for caching things way lower :)

See `quickcache` docstring for documentation on how to use it and what its features are.
I found http://stackoverflow.com/a/20530957/8207 which made me think that the performance gains are probably worth it in the short term and if we see memory issues on the workers we can disable it then