* plugin/transfer: only allow outgoing axfr over tcp

Return refused when the query comes in over udp.
No need to add a new test case as the current crop needed to be changed
to use TCP.

Fixes: #4450
Signed-off-by: Miek Gieben <miek@miek.nl>

* transfer tests: this needs tcp as well
Signed-off-by: Miek Gieben <miek@miek.nl>