C# fixes for #1472 core API cleanup

src/csharp/Grpc.Core.Tests/PInvokeTest.cs

src/csharp/Grpc.Core/Internal/AsyncCall.cs

src/csharp/Grpc.Core/Internal/AsyncCallBase.cs

src/csharp/Grpc.Core/Internal/AsyncCallServer.cs

src/csharp/Grpc.Core/Internal/CallSafeHandle.cs

src/csharp/Grpc.Core/Internal/Enums.cs

src/csharp/Grpc.Core/Internal/ServerSafeHandle.cs

src/csharp/Grpc.Core/Server.cs

src/csharp/ext/grpc_csharp_ext.c