Previously, there was code to either use select.select or select.poll,
depending on OS.  This changes it to use the selectors module, using
either SelectSelector or PollSelector, but sharing code otherwise.

The existing receive_udp() methods are only usable for receiving
responses, as they require an expected destination and check that the
message is from that destination.

This change makes the expected destination (and hence the check)
optional, and returns the address that the message was received from (in
the sync case, this is only done if no destination is provided, for
backwards compatibility).

New tests are added, which required adding generic getsockname() support
to the async backends.

- When no port is specified, pick the same port for UDP and TCP, so that
TCP fallback can be tested.

- Change handlers to get a single Request object instead of individual
parameters.  The Request object contains the message, peer, and
connection_type previously passed, and also adds the local address and
wire format message.  Additionally, it provides convenient properties
for accessing the question.