make ipv4 addresses trivially work with ipv6 by using AI_V4MAPPED
generate correct ipv6 urls
declare the _send_request interface as internal

they are not shared with outer request classes, and are more a relic of
the twisted code structure
crude multicast retransmission
don't explicitly set remote, that doesn't go well with hybrid stack approaches
Merge branch 'master' into for-readthedocs