Refactor package object model.

Source/MQTTnet/Implementations/PlatformAbstractionLayer.cs

@@ -1,20 +1,25 @@
-using System.Threading.Tasks;
+using System;
+using System.Threading.Tasks;
 
 namespace MQTTnet.Implementations
 {
     public static class PlatformAbstractionLayer
     {
-        public static Task CompletedTask
-        {
-            get
-            {
-#if NET452 
-                return Task.FromResult(0);
-#else
-                return Task.CompletedTask;
-#endif
-            }
-        }
+        public static readonly byte[] EmptyByteArray = new byte[0];
 
+#if NET452
+        public static Task CompletedTask => Task.FromResult(0);
+#else
+        public static Task CompletedTask => Task.CompletedTask;
+#endif
+
+        public static void Sleep(TimeSpan timeout)
+        {
+#if NET452 || NETSTANDARD2_0 || NETSTANDARD2_1 || NETCOREAPP3_1
+            System.Threading.Thread.Sleep(timeout);
+#else
+            Task.Delay(timeout).Wait();
+#endif
+        }
     }
 }