BUG/MEDIUM: mux-h2: do not close the connection on aborted streams

BUG/MEDIUM: mux-h2: do not close the connection on aborted streams

We used to rely on a hint that a shutw() or shutr() without data is an
indication that the upper layer had performed a tcp-request content reject
and really wanted to kill the connection, but sadly there is another
situation where this happens, which is failed keep-alive request to a
server. In this case the upper layer stream silently closes to let the
client retry. In our case this had the side effect of killing all the
connection.

Instead of relying on such hints, let's address the problem differently
and rely on information passed by the upper layers about the intent to
kill the connection. During shutr/shutw, this is detected because the
flag CS_FL_KILL_CONN is set on the connstream. Then only in this case
we send a GOAWAY(ENHANCE_YOUR_CALM), otherwise we only send the reset.

This makes sure that failed backend requests only fail frontend requests
and not the whole connections anymore.

This fix relies on the two previous patches adding SI_FL_KILL_CONN and
CS_FL_KILL_CONN as well as the fix for the connection close, and it must
be backported to 1.9 and 1.8, though the code in 1.8 could slightly differ
(cs is always valid) :

  BUG/MEDIUM: mux-h2: wait for the mux buffer to be empty before closing the connection
  MINOR: stream-int: add a new flag to mention that we want the connection to be killed
  MINOR: connstream: have a new flag CS_FL_KILL_CONN to kill a connection