refactoring the store

I'm making some good headway on rearranging the store to match the rough
API prepared in Stockholm. (Kinkie: where was that wiki you put up
again? I'll put the design notes up there...).

its fairly butt-ugly right now, as its a fusion of three separate APIs -
the squid 2.5 one, the incrementally factored 3.0 one, and the one I'm
now heading too.

Anyone interested in the code can see it in:
robertc@squid-cache.org--squid/squid--store-interface--3.0 in arch.

Cheers,
Rob