Use SDK-style project where possible (#629)

* 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.

Author: drieseng

appveyor.yml

@@ -1,14 +1,14 @@
-os: Visual Studio 2015
+os: Visual Studio 2019
 
 before_build:
-  - nuget restore src\Renci.SshNet.VS2015.sln
+  - nuget restore src\Renci.SshNet.VS2019.sln
 
 build:
-  project: src\Renci.SshNet.VS2015.sln
+  project: src\Renci.SshNet.VS2019.sln
   verbosity: minimal
 
 test_script:
 - cmd: >-
-    vstest.console /logger:Appveyor src\Renci.SshNet.Tests\bin\Debug\Renci.SshNet.Tests.dll /TestCaseFilter:"TestCategory!=integration&TestCategory!=LongRunning"
+    vstest.console /logger:Appveyor src\Renci.SshNet.Tests\bin\Debug\net40\Renci.SshNet.Tests.dll /TestCaseFilter:"TestCategory!=integration&TestCategory!=LongRunning"
 
-    vstest.console /logger:Appveyor src\Renci.SshNet.Tests.NET35\bin\Debug\Renci.SshNet.Tests.NET35.dll /TestCaseFilter:"TestCategory!=integration&TestCategory!=LongRunning"
+    vstest.console /logger:Appveyor src\Renci.SshNet.Tests\bin\Debug\net35\Renci.SshNet.Tests.dll /TestCaseFilter:"TestCategory!=integration&TestCategory!=LongRunning"

runtest.cmd

@@ -0,0 +1,3 @@
+rem vstest.console src\Renci.SshNet.Tests\bin\Debug\net40\Renci.SshNet.Tests.dll "/TestCaseFilter:TestCategory=Gert
+
+vstest.console src\Renci.SshNet.Tests\bin\Debug\net40\Renci.SshNet.Tests.dll /TestCaseFilter:"TestCategory!=integration&TestCategory!=LongRunning"