Merge pull request #968 from jtattermusch/csharp_refactoring

src/csharp/.gitignore

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

src/csharp/Grpc.Core/ChannelArgs.cs

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

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/AsyncCompletion.cs

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

...csharp/Grpc.Core/Internal/ClientStreamingInputObserver.cs

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

...sharp/Grpc.Core/Internal/ServerStreamingOutputObserver.cs

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

src/csharp/Grpc.Core/OperationFailedException.cs

src/csharp/Grpc.Core/ServerCallHandler.cs

src/csharp/Grpc.Core/Status.cs

src/csharp/Grpc.Core/Utils/Preconditions.cs

src/csharp/Grpc.Examples.MathClient/MathClient.cs

src/csharp/Grpc.Examples/MathExamples.cs