code style fixes based on StyleCop warnings

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

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

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

src/csharp/Grpc.Core.Tests/Properties/AssemblyInfo.cs

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

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

src/csharp/Grpc.Core/Call.cs

src/csharp/Grpc.Core/Calls.cs

src/csharp/Grpc.Core/Channel.cs

src/csharp/Grpc.Core/ChannelArgs.cs

src/csharp/Grpc.Core/ClientStreamingAsyncResult.cs

src/csharp/Grpc.Core/Credentials.cs

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/AsyncCompletion.cs

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

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

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

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