Extract the NWConnection code into StateManagedNWConnectionChannel

[Joannis]

Extract the NWConnection code into StateManagedNWConnectionChannel

Motivation:

When implementing #68, a lot of code ended up being identical between TCP and UDP. This PR paves the way to adding UDP, with very little overhead code. It moves the code that is used to represent NWConnection as a Channel into its own protocol, that can then be applied to both TCP and UDP implementations.

Modifications:

• Added StateManagedNWConnectionChannel that has all the logic for handling a NWConnection as a NIO.Channel.
• Added a new internal protocol NWOptionsProtocol that can read and write socket options. This is applied to NWProtocolTCP.Option
  • StateManagedNWConnectionChannel handles the common denominators between TCP and the future UDP implementaiton
• Added NWConnectionSubstate that can handle changes in the ChannelState.

Result:

This makes implementing UDP much simpler, and without redundant code.

[Joannis]

@Lukasa

[Joannis]

In hindsight, I do think it's valuable to also add a common implementation for listener channels, and add that to UDP right away as well. Give me a bit more time.

EDIT: It'll be a second PR.

[Joannis]

Updated now! I removed the lock from addressCache and moved it back into the getters and setters accessing the cache

[Joannis]

Updated @Lukasa , thanks for the quick reviews thus far. And have a nice weekend for now :)

[Joannis]

Ping @Lukasa

[Lukasa]

@swift-server-bot add to allowlist

[Lukasa]

To try to get this landed I took the liberty of just fixing the soundness violation myself.