Command queue

New: Adding multiple series will queue them instead of running all at once
New: Slower scheduled tasks won't be block others from running

src/NzbDrone.Core/HealthCheck/HealthCheckService.cs

@@ -94,7 +94,7 @@ namespace NzbDrone.Core.HealthCheck
 
         public void Execute(CheckHealthCommand message)
         {
-            PerformHealthCheck(c => message.Manual || c.CheckOnSchedule);
+            PerformHealthCheck(c => message.Trigger == CommandTrigger.Manual || c.CheckOnSchedule);
         }
     }
 }