store url rewriter stuff

I've started work on the store url rewriter stuff in Squid-HEAD.
The eventual aim is to be able to use a "canonicalised" URL form for store
lookups, working around the tricks used by CDNs and other websites to
distribute content and improve parallelism. Think google maps, youtube and
microsoft updates caching.

The work done in Squid-2-HEAD so far is just laying groundwork and slightly
reshuffling store_client.c code around - shifting the redirector code out into
an external file - as preparation for further work.

The current work shouldn't affect normal squid-2.HEAD behaviour as its not
right now passing anything to the storeurl rewriters, nor is it using the
request_t->storeurl string in any cache lookups. There's the possibility
that I break a lot of stuff in the store key manipulations so that will
be the very last thing I implement and only after plenty of other testing.

(Note: This work isn't being sponsored by anyone; I'm just fed up that
its not implemented..)

Thanks,

Adrian

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