BUG/MINOR: mux-h2: make sure to produce a log on invalid requests

BUG/MINOR: mux-h2: make sure to produce a log on invalid requests

As reported by Dominik Froehlich in github issue #1968, some H2 request
parsing errors do not result in a log being emitted. This is annoying
for debugging because while an RST_STREAM is correctly emitted to the
client, there's no way without enabling traces to find it on the
haproxy side.

After some testing with various abnormal requests, a few places were
found where logs were missing and could be added. In this case, we
simply use sess_log() so some sample fetch functions might not be
available since the stream is not created. But at least there will
be a BADREQ in the logs. A good eaxmple of this consists in sending
forbidden headers or header syntax (e.g. presence of LF in value).
Some quick tests can be done this way:

- protocol error (LF in value):
  curl -iv --http2-prior-knowledge -H "$(printf 'a:b\na')" http://0:8001/

- too large header block after decoding:
  curl -v --http2-prior-knowledge -H "a:$(perl -e "print('a'x10000)")"  -H "a:$(perl -e "print('a'x10000)")"  http://localhost:8001/

This should be backported where needed, most likely 2.7 and 2.6 at
least for a start, and progressively to other versions.