Fixes for C# cancellation support

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

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

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

...harp/Grpc.Core/Internal/BatchContextSafeHandleNotOwned.cs

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

src/csharp/Grpc.Core/Status.cs

src/csharp/Grpc.IntegrationTesting/InteropClient.cs

src/csharp/ext/grpc_csharp_ext.c