bug 1089375 - spin out drf_cached_reads

This pull request builds on top of changes in PR #10, only the last two changes are unique to this PR.  You may want to accept that one before code reviewing this request.

The "drf_cached_reads" app has been moved to a new repository:

https://github.com/jwhitlock/drf-cached-instances

The code was removed from the web-platform-compaq repo, and instead added as a requirement.  Additionally,  a technique used to remove duplicated code in the new project was brought into this project.