Merge pull request #6754 from jtattermusch/csharp_better_shutdown

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

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

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

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

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

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

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

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

src/csharp/Grpc.Core/Channel.cs

src/csharp/Grpc.Core/GrpcEnvironment.cs

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

src/csharp/Grpc.Core/Server.cs

src/csharp/tests.json