Caching / Storing conjure (and other?) output

[ChrisJefferson]

Conjure tries to cache models when they are generated. However, this code has some significant limitations:

• Not thread/process safe
• If you solve a different problem, it over-writes the first problem's cache.

It would be great to have a more general caching framework. This could cache:

• Generated Essence'
• Generated SAT / minion / etc models
• Answers to problems

These caches should be designed from the start to be process and thread safe. I suggest (but open to suggestions!) this cache runs as a seperate program, maybe using sqlite (or similar), where programs can query for cache values, store them, and read them back out.