In case of restarting it can happen that the new request is started
before the previous one is fully completed. This is because restart
is scheduled on SoupMessage::got-body that happens right before the
stream is closed and the IO finished. In HTTP/1.x this is possible if a
new request is made in got-body callback, for example. In HTTP/2 is
easier to reproduce because we emit SoupMessage::got-body right after
SoupClientInputStream::eof. This patch adds an intermediate state
SOUP_MESSAGE_REQUEUED set by soup_session_requeue_item(). The transition
to SOUP_MESSAGE_RESTARTING happens when the message IO completes to
ensure the restarted request starts when message no longer has an active IO.