BUG/MEDIUM: h3: do not overwrite interim with final response

BUG/MEDIUM: h3: do not overwrite interim with final response

An HTTP response may contain several interim response message prior (1xx
status) to a final response message (all other status codes). This may
cause issues with h3_resp_headers_send() called for response encoding
which assumes that it is only call one time per stream, most notably
during output buffer handling.

This commit fixes output buffer handling when h3_resp_headers_send() is
called multiple times due to an interim response. Prior to it, interim
response was overwritten with newer response message. Most of the time,
this resulted in error for the client due to QPACK decoding failure.
This is now fixed so that each response is encoded one after the other.

Note that if encoding of several responses is bigger than output buffer,
an error is reported. This can definitely occurs as small buffer are
used during header encoding. This situation will be improved by the next
patch.

This must be backported up to 2.6.