Stop using native callbacks for C#

src/csharp/Grpc.Core.Tests/Grpc.Core.Tests.csproj

...harp/Grpc.Core.Tests/Internal/CompletionQueueEventTest.cs

...Grpc.Core.Tests/Internal/CompletionQueueSafeHandleTest.cs

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

src/csharp/Grpc.Core/Grpc.Core.csproj

src/csharp/Grpc.Core/GrpcEnvironment.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/BatchContextSafeHandle.cs

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

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

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

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

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

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

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

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

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

src/csharp/Grpc.Core/Server.cs