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.