Support propagating ECN information on AddressedEnvelopes

[Lukasa]

It is possible that users of datagram channels may be interested in ECN information associated with the IP packets that carry the datagrams. It's possible to obtain this information from the OS and for us to propagate it through the channel, but today we do not.

The proposal for how to do so comes in multiple parts, each of which is probably a separate chunk of work:

• Add support for AddressedEnvelope to carry arbitrary metadata in a performant way

  This probably involves adding a metadata field to AddressedEnvelope that is usually empty, and only ever filled when necessary. This metadata will need to be keyed and typed. This is a straightforward Swift-level data structure problem, and requires no NIO-specific knowledge.

• Move datagram channels over to using recvmsg in the single-read case, instead of recvfrom. (Move DatagramChannel over to using recvmsg. #1473)

  This allows us to extract metadata from socket reads.

• Wire up extracting metadata of known types from recvmsg.

  Just a straightforward extension. We need to minimise allocations here, so we should aim to be careful about how we build this. Ideally we'll use the stack.

• Add a ChannelOption that allows users to request ECN information, and then make sure we extract and propagate that information.

• Update swift-nio-transport-services to support this new flag, as Network.framework automatically provides this information for us.

[PeterAdams-A]

Solved for swift-nio iin #1587, #1590, #1591, #1593 , #1594 and #1596.

I've made an issue in transport services - apple/swift-nio-transport-services#100 to track adding this support there.

[ovidiuurotaru]

Can we also read ECN flags using NIO?

[Lukasa]

Yes, this is implemented. Set ChannelOption.explicitCongestionNotification on your DatagramBootstrap to enable it.