Add a SQLite caching layer (for SPR)

[Aaron Straup Cope]

A side-effect of all the sqlite/extras work is that I think I have all the pieces to use SQLite databases that are generated for extras for generating the standard SPR responses too which in principle means the overall memory requirements will come down.

I am not convinced the savings won't be eaten up entirely by disk and other I/O stuff but one thing at a time...