Don't call getsockname on every packet

[maskit]

UnixUDPConnection should have a local copy of the local address so no need to bother Kernel.

[JosiahWI]

@maskit What's the reason for changing the socket storage type from sockaddr_in6 to sockaddr_storage?

[maskit]

Because it seemed more appropriate type, although sockkaddr_in6 is technically large enough for our use case.

[JosiahWI]

Thanks. It's nice not to waste resources calling getsockname when the DNSConnection already has a local copy. It looks like that local copy should always exist since it's created in UDPBind.

I'm sad to see the introduction of reinterpret_casts, but it seems sockaddr_storage is designed for this kind of purpose, and the casts are guaranteed by POSIX in most cases not to violate strict aliasing, which is a nice bonus even though we don't enforce strict aliasing requirements.

[maskit]

It looks like that local copy should always exist since it's created in UDPBind.

This may not be true on outgoing QUIC connections, although we currently don't support it.

[JosiahWI]

I think it's set on the QUIC connections as well.

[cmcfarlen]

Cherry-picked to v10.0.x