BUG/MINOR: h3: properly realloc buffer after interim response encoding

BUG/MINOR: h3: properly realloc buffer after interim response encoding

Previous commit fixes encoding of several following HTTP response
message when interim status codes are first reported. However,
h3_resp_headers_send() still was unable to interrupt encoding if output
buffer room was not sufficient. This case may be likely because small
buffers are used for headers encoding.

This commit fixes this situation. If output buffer is not empty prior to
response encoding, this means that a previous interim response message
was already encoded before. In this case, and if remaining space is not
sufficient, use buffer release mechanism : this allows to restart
response encoding by using a newer buffer. This process has already been
used for DATA and trailers encoding.

This must be backported up to 2.6. However, note that buffer release
mechanism is not present for version 2.8 and lower. In this case, qcs
flag QC_SF_BLK_MROOM should be enough as a replacement.