Fix an issue where under teleport v2 protocol, teleporting from regions in an line from A->B->C would not close region A when reaching C

The root cause was that v2 was only closing neighbour agents if the root connection also needed a close.
However, fixing this requires the neighbour regions also detect when they should not close due to re-teleports re-establishing the child connection.
This involves restructuring the code to introduce a scene presence state machine that can serialize the different add and remove client calls that are now possible with the late close of the
This commit appears to fix these issues and improve teleport, but still has holes on at least quick reteleporting (and possibly occasionally on ordinary teleports).
Also, has not been completely tested yet in scenarios where regions are running on different simulators

OpenSim/Region/ClientStack/Linden/Caps/EventQueue/Tests/EventQueueTests.cs

@@ -91,7 +91,7 @@ namespace OpenSim.Region.ClientStack.Linden.Tests
         public void RemoveForClient()
         {
             TestHelpers.InMethod();
-//            log4net.Config.XmlConfigurator.Configure();
+//            TestHelpers.EnableLogging();
 
             UUID spId = TestHelpers.ParseTail(0x1);
 

OpenSim/Region/ClientStack/Linden/UDP/LLClientView.cs

@@ -512,7 +512,13 @@ namespace OpenSim.Region.ClientStack.LindenUDP
                 // We still perform a force close inside the sync lock since this is intended to attempt close where
                 // there is some unidentified connection problem, not where we have issues due to deadlock
                 if (!IsActive && !force)
+                {
+                    m_log.DebugFormat(
+                        "[CLIENT]: Not attempting to close inactive client {0} in {1} since force flag is not set", 
+                        Name, m_scene.Name);
+
                     return;
+                }
 
                 IsActive = false;
                 CloseWithoutChecks();

OpenSim/Region/ClientStack/Linden/UDP/LLUDPServer.cs

@@ -1799,9 +1799,9 @@ namespace OpenSim.Region.ClientStack.LindenUDP
     
                 if (!client.SceneAgent.IsChildAgent)
                      client.Kick("Simulator logged you out due to connection timeout.");
-    
-                client.CloseWithoutChecks();
             }
+
+            m_scene.IncomingCloseAgent(client.AgentId, true);
         }
 
         private void IncomingPacketHandler()
@@ -2142,7 +2142,7 @@ namespace OpenSim.Region.ClientStack.LindenUDP
             if (!client.IsLoggingOut)
             {
                 client.IsLoggingOut = true;
-                client.Close();
+                m_scene.IncomingCloseAgent(client.AgentId, false);
             }
         }
     }

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

@@ -200,7 +200,7 @@ namespace OpenSim.Region.ClientStack.LindenUDP.Tests
         public void TestLogoutClientDueToAck()
         {
             TestHelpers.InMethod();
-//            TestHelpers.EnableLogging();
+            TestHelpers.EnableLogging();
 
             IniConfigSource ics = new IniConfigSource();
             IConfig config = ics.AddConfig("ClientStack.LindenUDP");