MEDIUM: h3: Use flags of HTX blocks to detect end of message during sending

MEDIUM: h3: Use flags of HTX blocks to detect end of message during sending

When a message is sent, the end of the message is now detected by testing
flags of each HTX blocs. To do the API was slightly changed. The "fin" flag
is not longer set in the main loop in h3_snd_buf() function. Instead, the
flag is set in functions consumming the HTX blocks (h3_req_headers_send,
h3_resp_headers_send...).

H3_SF_SENT_INTERIM flag is no longer tested. An interim message cannot have
EOM set. A BUG_ON() may possibly be added. In addition, the HTX message is
not check for emptiness. Here again a BUG_ON() may be added.