State Managed Listeners

[Joannis]

Extract the NWListener code into StateManagedListenerChannel

Motivation:

When implementing #68, a lot of code ended up being identical between TCP and UDP. Like #174, this PR paves the way to adding UDP, with very little overhead code. It moves the code that is used to represent NWListener as a Channel into its own protocol, that can then be applied to both TCP and UDP implementations.

Modifications:

• Added StateManagedListenerChannel that has all the logic for handling a NWConnection as a NIO.Channel.
• Made the AcceptHandler generic for use with both TCP and UDP

Result:

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

Notes

This PR works off #174 , and should therefore be viewed as a diff based on #174 to get a clean picture.

[Lukasa]

@Joannis any chance you can rebase this on the new main for easier review?

[Joannis]

Will do @Lukasa , doing so now

[Joannis]

@Lukasa ready for review again