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.
related to my comment on https://github.com/dimagi/dimagi-utils/pull/237 - are we sure this is a good idea? we already run very memory-heavy and it seems like abusing this could get us into trouble (as opposed to memcached or redis which presumably have good memory management mechanisms automatically built into them).