generalized search caching+discussion

a few observations:

• Hard for me to tell so far how generalizable the "Test" part is; would be good to work out some examples.
• Worth noting that no db queries are involved in the tests (provided the cache is populated)
• Even if we don't generalize the capacity to auto-generate a test from WQL, this general structure could be pretty useful in moving the rationale of cache clearing away from the clearers and toward the clearees (which would seem to be a more reusable pattern).
• For example, CalculatedValues would not be a CachedSearch card, but it could certainly add code to a "cache_clear hash" and function largely as outlined above.
• Would be cool to explore how this might work in caching "contributions" on WikiRate as one of the next steps.