[core] HTTP/2 GOAWAY after timeout before read (fixes #3102)

HTTP/2 send GOAWAY soon after keep-alive timeout, before potentially
reading new stream requests, which will then have to be reset.

x-ref:
  "Chrome gives random net::ERR_HTTP2_PROTOCOL_ERROR"
  https://redmine.lighttpd.net/issues/3102

src/connections.c

@@ -1460,6 +1460,7 @@ static void connection_check_timeout (connection * const con, const unix_time64_
                               con->fd);
                 }
                 connection_set_state(r, CON_STATE_RESPONSE_END);
+                con->is_readable = 0;
                 changed = 1;
             }
         }