adapt C# to new cq_create API

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

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

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

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

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

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

src/csharp/ext/grpc_csharp_ext.c