Move frame loop entirely within Scene.Update() for better future performance analysis and stat accuracy.

Update() now accepts a frames parameter which can control the number of frames updated.
-1 will update until shutdown.
The watchdog updating moves above the maintc recalculation for any required sleep since it should be accounted for within the frame.

OpenSim/Region/ClientStack/Linden/UDP/Tests/MockScene.cs

@@ -50,7 +50,7 @@ namespace OpenSim.Region.ClientStack.LindenUDP.Tests
             m_regStatus = RegionStatus.Up;
         }
         
-        public override void Update() {}
+        public override void Update(int frames) {}
         public override void LoadWorldMap() {}
         
         public override ISceneAgent AddNewClient(IClientAPI client, PresenceType type)