Slixmpp behaves differently when resolving host names, whether aiodns
is used or not. With aiodns only DNS is used, while without
asyncio.loop.getaddrinfo() is used instead, which utilizes the Name
Service Switch (NSS) to resolve host names by other means (hosts-file,
mDNS, ...) as well.
To unify the behavior, this replaces the use of
aiodns.DNSResolver().query() with
aiodns.DNSResolver().gethostbyname(). This makes the behavior
resolving host names more consistent between using aiodns or not, as
both now honor the NSS configuration and removes the need for the
previously existing workaround to resolve localhost.