Describe the bug
Opening and closing (async) a device client, in a using statement never shuts down properly.

Expected behavior
DotNetty.Common.Concurrency.SingleThreadEventExecutor.PollTask threads stop, and program exits

Actual behavior (include Exception or Stack Trace)
Program does not exit,
8 DotNetty.Common.Concurrency.SingleThreadEventExecutor.PollTask threads run pretty much forever

To Reproduce
Steps to reproduce the behavior (include a code snippet, screenshot, or any additional information that might help us reproduce the issue)

1. .net console app, 4.8
2. install nuget package "Microsoft.Azuer.Devices.Client" 1.38.0
3. run console, for however long or short you want
4. type anything, hit enter
5. program never exits

        class Program
	{
         	private static string _connection_string = $"HostName={url};DeviceId={the_id};SharedAccesskey={key}";// fill your in
		public static bool is_running = false;
		static void Main(string[] args)
		{

			is_running = true;
			new System.Threading.Thread(new System.Threading.ThreadStart(thread_method)).Start();
			Console.WriteLine("enter to exit");
			String line = Console.ReadLine();
			is_running = false;
		}
		public static async void thread_method()
		{
			using (var _deviceClient = DeviceClient.CreateFromConnectionString(_connection_string, TransportType.Mqtt))
			{
				await _deviceClient.OpenAsync();
				while (is_running) System.Threading.Thread.Sleep(500);
				await _deviceClient.CloseAsync();
			}
            }
}

Environment:

• Name and version of the Library package used: "Microsoft.Azuer.Devices.Client" 1.38.0
• Hosting platform or OS and .NET runtime version:

Windows 10

Target Framework 4.8

Microsoft.AspNetCore.All 2.1.12 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.26 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.12 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.26 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.13 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.18 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.9 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.0.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.0.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.12 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.26 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.13 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.18 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 3.1.13 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 3.1.18 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.4 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.9 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

• IDE and version : Visual Studio 16.11.2