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.
Tested by running this on staging https://gist.github.com/dannyroberts/2f63e276d3a2e353017a. I've seen forms that are in the 90K's and found one in my local commcare instance that's 92K so I used that number to test. I got

```python
>>> print time_locmem_ms(n)
0.259321
>>> print time_memcached_ms(n)
20.563619
```
Those numbers are in ms per 92K payload. Looks like locmem's about 100x faster, but they're both pretty fast. In the case of the other PR (about caching xform attachments), there were ~15 repeat fetches of that type on the pageload I was looking at, so this saves 15 * (20.5 - .26) = 300ms, which isn't nothing, though it's currently only about a tenth of the pageload. I imagine that for something that touches all forms, like making a build, this could make a much bigger difference.

I think the conclusion from this (if I'm not missing something and it's representative of the real production situation) is that it's significant but not worth a great amount of trouble.

Luckily it's the easiest thing in the world (~1 line change) to change the `quickcache` default to only use memcached, so if we have problems we can easily experiment with turning that on and off without touching any of the client code.