Add queue with async support to free up threads.

Source/MQTTnet/Internal/AsyncBlockingQueue.cs

@@ -0,0 +1,39 @@
+using System;
+using System.Collections.Concurrent;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace MQTTnet.Internal
+{
+    public sealed class AsyncQueue<TItem> : IDisposable
+    {
+        private readonly SemaphoreSlim _semaphore = new SemaphoreSlim(0);
+        private readonly ConcurrentQueue<TItem> _queue = new ConcurrentQueue<TItem>();
+
+        public void Enqueue(TItem item)
+        {
+            _queue.Enqueue(item);
+            _semaphore.Release();
+        }
+
+        public async Task<TItem> DequeueAsync(CancellationToken cancellationToken)
+        {
+            while (true)
+            {
+                await _semaphore.WaitAsync(cancellationToken).ConfigureAwait(false);
+
+                cancellationToken.ThrowIfCancellationRequested();
+
+                if (_queue.TryDequeue(out var item))
+                {
+                    return item;
+                }
+            }
+        }
+
+        public void Dispose()
+        {
+            _semaphore?.Dispose();
+        }
+    }
+}