SoupSession: continue reorg, remove remaining signal-based flow control

SoupSession: continue reorg, remove remaining signal-based flow control

Make the sessions more state-machiney and fix up
soup_session_cancel_message() by having it set the message to the
(new) FINISHING state and letting the session implementation pick it
up from there. Remove the remaining "finished" and "restarted" signal
handlers; move the base session's "finished" handler to
"soup_session_unqueue_item" and call that from the right places in the
subclasses.