Reinsert a 2000ms delay before closing a no longer required agent on the source region after teleport to resolve Imprudence teleport problems.

Viewers 1 and 3 are fine with doing this immediately.  However, Imprudence has a small delay (<200ms, >500ms) after receiving the AgentCompleteMovement reply packet on the destination region before regarding that region as the currnet region.
If Imprudence receives a DisableSimulator in this period, it quits.
We are not restoring the full 5000ms delay since this brings back a bug where teleports permanently fail if an avatar tries to teleport back too quickly.
This commit also sends the AgentCompleteMovement packet to the client before telling the source region to release its old agent, in order to further cut down any possibility of the DisableSimulator being recieved before the AgentMovementComplete.

OpenSim/Region/CoreModules/Framework/EntityTransfer/EntityTransferModule.cs

@@ -30,7 +30,6 @@ using System.Collections.Generic;
 using System.Net;
 using System.Reflection;
 using System.Threading;
-
 using OpenSim.Framework;
 using OpenSim.Framework.Capabilities;
 using OpenSim.Framework.Client;
@@ -589,7 +588,13 @@ namespace OpenSim.Region.CoreModules.Framework.EntityTransfer
 
                 if (NeedsClosing(sp.DrawDistance, oldRegionX, newRegionX, oldRegionY, newRegionY, reg))
                 {
-//                    Thread.Sleep(5000);
+                    // We need to delay here because Imprudence viewers, unlike v1 or v3, have a short (<200ms, <500ms) delay before
+                    // they regard the new region as the current region after receiving the AgentMovementComplete
+                    // response.  If close is sent before then, it will cause the viewer to quit instead.
+                    // However, if this delay is longer, then a viewer can teleport back to this region and experience
+                    // a failure because the old ScenePresence has not yet been cleaned up.
+                    Thread.Sleep(2000);
+
                     sp.Close();
                     sp.Scene.IncomingCloseAgent(sp.UUID);
                 }

OpenSim/Region/Framework/Scenes/ScenePresence.cs

@@ -1202,9 +1202,19 @@ namespace OpenSim.Region.Framework.Scenes
 
             bool flying = ((m_AgentControlFlags & AgentManager.ControlFlags.AGENT_CONTROL_FLY) != 0);
             MakeRootAgent(AbsolutePosition, flying);
+            ControllingClient.MoveAgentIntoRegion(m_scene.RegionInfo, AbsolutePosition, look);
+
+//            m_log.DebugFormat("[SCENE PRESENCE] Completed movement");
 
             if ((m_callbackURI != null) && !m_callbackURI.Equals(""))
             {
+                // We cannot sleep here since this would hold up the inbound packet processing thread, as
+                // CompleteMovement() is executed synchronously.  However, it might be better to delay the release
+                // here until we know for sure that the agent is active in this region.  Sending AgentMovementComplete
+                // is not enough for Imprudence clients - there appears to be a small delay (<200ms, <500ms) until they regard this
+                // region as the current region, meaning that a close sent before then will fail the teleport.
+//                System.Threading.Thread.Sleep(2000);
+
                 m_log.DebugFormat(
                     "[SCENE PRESENCE]: Releasing {0} {1} with callback to {2}",
                     client.Name, client.AgentId, m_callbackURI);
@@ -1213,9 +1223,6 @@ namespace OpenSim.Region.Framework.Scenes
                 m_callbackURI = null;
             }
 
-//            m_log.DebugFormat("[SCENE PRESENCE] Completed movement");
-
-            ControllingClient.MoveAgentIntoRegion(m_scene.RegionInfo, AbsolutePosition, look);
             ValidateAndSendAppearanceAndAgentData();
 
             // Create child agents in neighbouring regions