BUG/MINOR: h3: do no crash on forwarding multiple chained response

BUG/MINOR: h3: do no crash on forwarding multiple chained response

h3_resp_headers_to_htx() is the function used to convert an HTTP/3
response into a HTX message. It was introduced on this release for QUIC
backend support.

A BUG_ON() would occur if multiple responses are forwarded
simultaneously on a stream without rcv_buf in between. Fix this by
removing it. Instead, if QCS HTX buffer is not empty when handling with
a new response, prefer to pause demux operation. This is restarted when
the buffer has been read and emptied by the upper stream layer.

No need to backport.