Allow shutdown of underlying Socket

### Description

We use the debugger in the integration-test suite (to test various stuff) and in order to make it work properly we need to shutdown the underlying socket.
To do this we need:
- https://github.com/zalmoxisus/remotedev/pull/17
- Some changes in this Repository to use this new `destroy` function.

While technically this requires https://github.com/elmish/elmish/issues/162
We have implemented it by "abusing" the `init` function in the following way: If `init` is called with a parameter of `null` we assume this is a program shutdown, which is good enough for our testing scenario.

This issue is a remainder and a starting point for discussion.
Is there interest in bringing these changes to this repository, maybe even depending on my fork of `remotedev`?

I spoke about this issue in the past [here](https://github.com/elmish/debugger/pull/21#issuecomment-466520210) and [here](https://github.com/elmish/debugger/pull/22#issuecomment-466527648)