BUG/MINOR: mux-h2: do not stop outgoing connections on stopping

BUG/MINOR: mux-h2: do not stop outgoing connections on stopping

There are reports of a few "SC" in logs during reloads when H2 is used
on the backend side. Christopher analysed this as being caused by the
proxy disabled test in h2_process(). As the comment says, this was done
for frontends only, and must absolutely not send a GOAWAY to the backend,
as all it will result in is to make newly queued streams fail.

The fix consists in simply testing the connection side before deciding
to send the GOAWAY.

This may be backported as far as 2.0, though for whatever reason it seems
to manifest itself only since 2.2 (probably due to changes in the outgoing
connection setup sequence).