client side streams #2

Ok, this is now a ready-to-go first slice of the streams code. I
*really* want to flush this from my buffer before it gets too big or
more HEAD changes to client_side accrue that will need management.

There will be more work done in the branch, but for HEAD, where every
client side stream is the trivial case (no data intermediaries) the
current code is able to handle everything.

Architecturally the code changes:
* client_side no longer requests anything from the store - it all goes
through the stream.
* pipelined and persistent connection logic has been tidied up to
be more robust.
* client_side_reply handles all of the store logic.
* client_side_request handles all of the client side ACL's, redirection
and the like.
* client_side.c handles FD's pipelining, persistent connections.
* clientHttpRequest has been stripped down, and is a much leaner
structure now.
* client_side_reply forms the basis for a replacement cache/store
interface that accepts a request and returns a reply - managing vary,
range and the rest under the hood.

Things that are out of scope for me today:
* client body uploads

Please please please give this a go!

Rob