[MERGE-3.0] Bug #2001: Handle response headers >4KB

This patch makes Squid deal with responses having more than 4KB of
responseline + headers.

- Make the HTTP protocol handler continue reading the response until all
is received. This involves growing the header buffer as needed until the
header fits (or limit reached). Previously the buffer based flow control
made Squid stop reading the response when reaching 4KB

- makes client_side_reply clone the already parsed HTTP response and
skip over the ascii HTTP header received from the store, similar to the
change Adrian did earlier in Squid-2 but for other reasons.