MINOR: haload: always track and resend cookies per user

MINOR: haload: always track and resend cookies per user

Capture every Set-Cookie header from a response, and send them back
as a Cookie header on that user's later requests. This is always on,
not behind an option: haload should behave like a real client here.

Cookies are kept per user, not per URL: they stay across a
reconnect, and are shared between the URLs that user hits, the same
way a real browser would.

http_extract_cookie_value() (src/http.c) is reused to parse the value
part (it already handles quotes and spaces correctly); only finding
the name comes first, since that function needs the name to already
be known.