Ignore, and don't save to memory, messages larger than 20MB (temporary)
Add support for IPv6

It will now listen on an IPv6 socket if possible or fall back to IPv4
if that doesn't work. It will no longer filter out all IPv6 addresses
and instead it will only filter out those that point to the local
network.

It looks like the DNS bootstrapping should just automatically work
because getaddrinfo already returns IPv6 addresses from the AAAA
record.

In order to convert from the ASCII representation of IPv6 addresses
and back we need inet_ntop and inet_pton. Python 2 doesn't currently
provide these for Windows so instead this patch provides a hot patch
to the socket module which wraps WSAStringToAddress and
WSAAddressToString using ctypes.