Expose the SOA query transport type used before/during XFR

Expose the SOA query transport type used before/during XFR

Add a new field in the incoming zone transfers section of the
statistics channel to show the transport used for the SOA request.

When the transfer is started beginning from the XFRST_SOAQUERY state,
it means that the SOA query will be performed by xfrin itself, using
the same transport. Otherwise, it means that the SOA query was already
performed by other means (e.g. by zone.c:soa_query()), and, in that
case, we use the SOA query transport type information passed by the
'soa_transport_type' argument, when the xfrin object was created.