Changed namespace from Google.GRPC to Grpc, sorted using statements, minor refactorings

src/csharp/GrpcApi/MathExamples.cs

src/csharp/GrpcApi/MathGrpc.cs

src/csharp/GrpcApi/MathServiceImpl.cs

src/csharp/GrpcApi/TestServiceGrpc.cs

src/csharp/GrpcApiTests/MathClientServerTests.cs

src/csharp/GrpcCore/Call.cs

src/csharp/GrpcCore/Calls.cs

src/csharp/GrpcCore/Channel.cs

src/csharp/GrpcCore/ClientStreamingAsyncResult.cs

src/csharp/GrpcCore/GrpcEnvironment.cs

src/csharp/GrpcCore/Internal/AsyncCall.cs

...sharp/GrpcCore/Internal/BatchContextSafeHandleNotOwned.cs

src/csharp/GrpcCore/Internal/CallSafeHandle.cs

src/csharp/GrpcCore/Internal/ChannelSafeHandle.cs

src/csharp/GrpcCore/Internal/ClientStreamingInputObserver.cs

src/csharp/GrpcCore/Internal/CompletionQueueSafeHandle.cs

src/csharp/GrpcCore/Internal/Enums.cs

src/csharp/GrpcCore/Internal/GrpcThreadPool.cs

src/csharp/GrpcCore/Internal/SafeHandleZeroIsInvalid.cs

src/csharp/GrpcCore/Internal/ServerSafeHandle.cs