Don't block task queue for queued update task when long running tasks queued

Fixes #1818

Co-Authored-By: Mark McDowall <markus101@users.noreply.github.com>

src/NzbDrone.Core/Indexers/RssSyncCommand.cs

@@ -5,5 +5,7 @@ namespace NzbDrone.Core.Indexers
     public class RssSyncCommand : Command
     {
         public override bool SendUpdatesToClient => true;
+
+        public override bool IsLongRunning => true;
     }
 }