* Change Renci.SshNet project into SDK-style project supporting .NET 3.5, .NET 4.0; .NET Standard 1.3, .NET Standard 2.0.
* Remove .NET 3.5 project.
* Remove .NET 3.5 test project.
* Refactor away .NET 3.5 and .NET 4.0 specific source files.
* Remove .NET Core project.
* Add VS 2019 solution.
* Use VS 2019 image and solution file in AppVeyor.
* Run tests against multi-target framework test project.
* No need to shutdown socket when client disconnects.