#31 Store cache in a database

[Bruce Merry]

Using something like sqlite would have a number of advantages:
- One big file instead of hundreds of little files
- Easier to drop just old versions
- No need to mess about with md5 sums and base64 encoding
- Possibly easier to debug, since one can use command-line tools to query the cache
- On-the-fly tuning would become viable, since locking could be handled portably
- Potentially faster repeated lookups, if things are in the page cache

The obvious downside is an extra dependency.

There should probably be one table per (algorithm,version) pair since this will determine the schema.