Rework some of the SoupAuthNegotiate internals

There are several problems with the current state. The main problem is
that the SoupMessage can outlive the SoupAuthNegotiate object and if the
SoupMessage signals handlers are active then they could be called with invalid
SoupAuthNegotiate object. To avoid that use the g_signal_connect_data() and
increase the reference on the SoupAuthNegotiate object. Also rework how
we are connecting the 'got_headers' signal handler to the SoupMessage object, so
they are really connected only once, even if the GSS mechanism involves
multiple rounds.

The whole concept of how we are working with the
SoupAuthNegotiateConnectionState is also wrong. When the connection state is
created it's saved to the private structure and then accessed from
there. The problem is that another state for different message could be
created in the mean time and that one would overwrite the currently set (or if
one would be freed then it would erase the one that is currently set). To solve
this expose the SoupConnectionAuth's get_connection_state_for_message() and
call it when we need the connection state object.