If the connection attempt succeeded, reserve the connection before

If the connection attempt succeeded, reserve the connection before

        * libsoup/soup-session.c (connect_result): If the connection
        attempt succeeded, reserve the connection before releasing
        host_lock. Otherwise, another thread might find it in the
        connection pool before the caller can queue a message on it.
        #60693

        * libsoup/soup-session-async.c (got_connection): Call
        soup_connection_release(), since we don't have a specific message
        in mind for the connection, so we need it to be considered idle.

        * libsoup/soup-connection.c (soup_connection_release): New
        function, to undo a soup_connection_reserve().
        (soup_connection_send_request, soup_connection_reserve,
        soup_connection_authenticate, soup_connection_reauthenticate):
        Document these