BUG/MEDIUM: h2: never leave pending data in the output buffer on close

BUG/MEDIUM: h2: never leave pending data in the output buffer on close

We currently don't process trailers on H2, but this has an impact : on
chunked HTTP/1 responses, we decide to emit the ES bit once we see the
0CRLF. From this point the stream switches to the CLOSED state, which
aborts processing of the remaining bytes. Thus the extra CRLF which ends
trailers is not processed and remains in the buffer. This prevents the
stream from being notified about end of transmission, which in turn keeps
the mux busy and prevents the connection from quitting.

The case of the trailers is not the root cause of this issue, though it
is what triggers it. The root cause is that upon error and/or close, once
we know we're not going to process any more data, we must absolutely flush
any remaining bytes from the output buffer, otherwise there is no way the
stream can quit. This is what this patch does.

It looks very likely related to the issues reported and debugged by
Janusz Dziemidowicz and Milan Petruzelka.

One way to reproduce it is to chain two proxies with the last one emitting
chunked data (typically using the stats page) :

    global
        stats socket /tmp/sock1 mode 666 level admin
        stats timeout 1h
        tune.ssl.default-dh-param 1024
        tune.bufsize 16384

    defaults
        mode http
        timeout connect 4s
        timeout client 10s
        timeout server 20s

    listen px1
        bind :4443 ssl crt rsa+dh2048.pem npn h2 alpn h2
        server s1 127.0.0.1:4445

    listen px2
        bind :4444 ssl crt rsa+dh2048.pem npn h2 alpn h2
        bind :4445
        stats uri /

Then use curl to fetch the stats through px1 :

    curl --http2 -k "https://127.0.0.1:4443/"

When curl is sent to the first one, "show sess" issued to the CLI will
show a remaining session during the client timeout. When curl is aimed at
port 4444 (px2), there is no such remaining session.

This fix needs to be backported to 1.8.