Refactor subscriptions manager

Frameworks/MQTTnet.NetStandard/Internal/AsyncLock.cs

@@ -0,0 +1,26 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace MQTTnet.Internal
+{
+    public sealed class AsyncLock : IDisposable
+    {
+        private readonly SemaphoreSlim _semaphore = new SemaphoreSlim(1, 1);
+
+        public Task EnterAsync(CancellationToken cancellationToken)
+        {
+            return _semaphore.WaitAsync(cancellationToken);
+        }
+
+        public void Exit()
+        {
+            _semaphore.Release();
+        }
+
+        public void Dispose()
+        {
+            _semaphore?.Dispose();
+        }
+    }
+}