migration to new C API

src/csharp/GrpcApi/MathGrpc.cs

src/csharp/GrpcApi/TestServiceGrpc.cs

src/csharp/GrpcCore/Calls.cs

src/csharp/GrpcCore/GrpcCore.csproj

src/csharp/GrpcCore/Internal/AsyncCall.cs

src/csharp/GrpcCore/Internal/BatchContextSafeHandle.cs

src/csharp/GrpcCore/Internal/CallSafeHandle.cs

src/csharp/GrpcCore/Internal/ClientStreamingInputObserver.cs

src/csharp/GrpcCore/Internal/CompletionQueueSafeHandle.cs

src/csharp/GrpcCore/Internal/Event.cs

src/csharp/GrpcCore/Internal/GrpcThreadPool.cs

src/csharp/GrpcCore/Internal/SafeHandleZeroIsInvalid.cs

src/csharp/GrpcCore/Internal/ServerSafeHandle.cs

...csharp/GrpcCore/Internal/ServerStreamingOutputObserver.cs

src/csharp/GrpcCore/Server.cs

src/csharp/GrpcCore/ServerCallHandler.cs

src/csharp/GrpcCoreTests/ClientServerTest.cs

src/csharp/ext/grpc_csharp_ext.c