Set the subscriber message delivery task to long running. (#140)

This helps ensure the message delivery task gets enough resources to perform.  Some systems may not dedicate enough resources, and since there is the SubscriberDeliveryTaskCount option available, this can be long running.

NATS.Client/AsyncSub.cs

@@ -81,7 +81,8 @@ internal void enableAsyncProcessing()
         {
             if (ownsChannel && msgFeeder == null)
             {
-                msgFeeder = new Task(() => { conn.deliverMsgs(mch); });
+                msgFeeder = new Task(() => { conn.deliverMsgs(mch); },
+                    TaskCreationOptions.LongRunning);
                 msgFeeder.Start();
             }
             started = true;