BUG/MINOR: mux-h1: Don't delay send if message with c-l was fully sent

BUG/MINOR: mux-h1: Don't delay send if message with c-l was fully sent

In the H1 multiplexer, if a message was fully sent considering the specified
content-length, the MSG_MORE flag must be unset, regardless the upper layer
said. This should avoid a extra latency of 200ms when this happens while the
HTX EOM flag is not set on the message. This may happen for H2/H3 messages
with trailers or if the END_OF_STREAM was not reported yet on H2/H3 side. In
both cases, on H1 side, no more data will be sent (trailers will be
dropped). So we must never delay the send.

This patch should fix the issue #3447. It must be backported to all
supported versions.