BUG/MEDIUM: stconn: Don't forward channel data if input data must be filtered

BUG/MEDIUM: stconn: Don't forward channel data if input data must be filtered

Once data are received and placed in a channel buffer, if it is possible,
outgoing data are immediately forwarded. But we must take care to not do so
if there is also pending input data and a filter registered on the
channel. It is especially important for HTX streams because the HTX may be
altered, especially the extra field. And it is indeed an issue with the HTTP
compression filter and the H1 multiplexer. The wrong chunk size may be
announced leading to an internal error.

This patch should fix the issue #2530. It must be backported to all stable
versions.