Introduce and refine SocketDevice

[MichaelWedel]

Experiments with a device that does real communication instead of simulation look promising. I think it's worthwhile spending some time trying to improve the design (or actually design something in the first place) so that it becomes easy to create such devices.

Owen pointed out that the name SocketDevice might not be appropriate, but I can't come up with a better name, so suggestions are welcome.

This is rather low on the list of priorities, so I'm putting the backlog label on it.

[MichaelWedel]

@mkoennecke has expressed interest in this aspect of the framework during the ECP workshop. One of the things he pointed out was that a proper queue is required. @MikeHart85 if I understood correctly, this sort of thing should be much easier with asynchat. Maybe we can take a look at this early next year.

[MichaelWedel]

As discussed earlier this week, this could likely not be a device but an adapter, although in "the other direction". In principle the currently existing poll_map is already somewhat similar to the bindings in StreamAdapter, it should be more like Var and Cmd.