Refactor xfr. (#1122)

Refactor xfr. (#1122)

* Refactor xfr.

Internally refactors the zone transfer code to separate the message
processing from the socket management, allowing the (internal) callers
to pass a socket in.  This should allow a future interface that accepts
a socket, which would mean that xfr over DoT would just work, and xfr
over DoQ would be closer to working.

Adds some necessary functionality to the asyncbackend Socket class to
allow the async zone transfer code to be more similar to the sync code
(specifically, adds a type field to Socket, and updates the trio backend
to connect UDP sockets when requested).

In asyncquery.py, reorder the inbound_xfr() and quic() methods for
consistency.

* Run black.

* Fix typing.