BUG/MINOR: http-htx: Don't by-pass HTX API when merging cookie values

BUG/MINOR: http-htx: Don't by-pass HTX API when merging cookie values

http_cookie_merge() function is responsible to add a cookie header and merge
all values from a list. However, it was performed by appending values by
hand, using the pointer to the header value and changing the block
length. This was totally by-passing the HTX API.

For now, there is now bug because the function is called by h2 and quic
muxes when a HTTP message is parsed. And the cookie header is the last one
inserted. The HTX message is never fragmented and data from other blocs
cannot be overwritten. But, it could be an issue if it is called in another
context, from the HTTP analysis for instance.

To fix the issue, the function now relies on htx_replace_blk_value()
function to add the value separator first and then a cookie value.

This patch must be backported as far as 2.6.