refactor: rename Scene.IncomingCloseAgent() to CloseAgent() in order to make it clear that all non-clientstack callers should be using this rather than RemoveClient() in order to step through the ScenePresence state machine properly.

Adds IScene.CloseAgent() to replace RemoveClient()

OpenSim/Region/ScriptEngine/Shared/Api/Implementation/OSSL_Api.cs

@@ -2964,7 +2964,7 @@ namespace OpenSim.Region.ScriptEngine.Shared.Api
                         sp.ControllingClient.Kick(alert);
 
                     // ...and close on our side
-                    sp.Scene.IncomingCloseAgent(sp.UUID, false);
+                    sp.Scene.CloseAgent(sp.UUID, false);
                 }
             });
         }