Make worker source compatible with .NET Core

[nguerrera]

First step towards running tests on CoreCLR. This makes it so that the worker can successfully compile against .NET Core packages, but leaves the actual project targeting intact.

• Work around lack of Environment.Exit
• Eliminate dependency on binary serializer
• Close -> Dispose
• Inline NamedPipeServerStream.MaxAllowedServerInstances = -1

@Pilchie @jaredpar

[jaredpar]

:+1: