Merge pull request #6640 from jtattermusch/csharp_assorted_cleanup

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

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

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

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

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

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

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

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

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

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

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

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

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

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