cache get_questions to make navigating app builder faster

cache includes hash of the form content, so invalidation is not
necessary
lazy_fetch_attachment should not throw KeyError
add quickcache and use for get_questions instead
use quickcache on the builds config doc
use quickcache to prevent config docs

use instead of cache_core.cached_open_doc
so it is cached even if the doc lookup 404s
add tests for quickcache

also split corehq/util/tests.py into two files under corehq/util/tests/
without otherwise changing those tests
quickcache fix typo
add tests for quickcache edge cases in key generation
Merge pull request #5032 from dimagi/caching-speedups

Quickcache