Bug #2001: Fails to forward responses where headers >4KB

This changes reply processing to clone the already parsed reply header
instead of trying to parse it yet another time. This allows us to seek
over the header contents and that way get away with just having a 4KB
buffer..

In the long run the store client API will be changed to provide a clean
split between headers and body, providing headers in parsed form (including
intermediary 1xx messages) and then the body starting at offset 0. This
change is a first step along that path but without changing the store client
api as such, with the intention of being merged into 3.0.