Start locking entire add/remove operations on an IScenePresence.AttachmentsSyncLock object

Attach and detach packets are processed asynchronously when received from a viewer.
Bugs like http://opensimulator.org/mantis/view.php?id=5644 indicate that in some situations (such as attaching/detaching entire folders of objects at once), there are race conditions between these threads.
Since multiple data structures need to be updated on attach/detach, it's not enough to lock the individual collections.
Therefore, this commit introduces a new IScenePresence.AttachmentsSyncLock which add/remove operations lock on.

OpenSim/Region/Framework/Interfaces/IAttachmentsModule.cs

@@ -87,6 +87,15 @@ namespace OpenSim.Region.Framework.Interfaces
         /// <returns>The scene object that was attached.  Null if the scene object could not be found</returns>
         ISceneEntity RezSingleAttachmentFromInventory(IClientAPI remoteClient, UUID itemID, uint AttachmentPt);
 
+        /// <summary>
+        /// Rez an attachment from user inventory and change inventory status to match.
+        /// </summary>
+        /// <param name="sp"></param>
+        /// <param name="itemID"></param>
+        /// <param name="AttachmentPt"></param>
+        /// <returns>The scene object that was attached.  Null if the scene object could not be found</returns>
+        ISceneEntity RezSingleAttachmentFromInventory(ScenePresence sp, UUID itemID, uint AttachmentPt);
+
         /// <summary>
         /// Rez multiple attachments from a user's inventory
         /// </summary>

OpenSim/Region/Framework/Interfaces/IScenePresence.cs

@@ -60,6 +60,14 @@ namespace OpenSim.Region.Framework.Interfaces
         /// </remarks>
         AvatarAppearance Appearance { get; set; }
 
+        /// <summary>
+        /// The AttachmentsModule synchronizes on this to avoid race conditions between commands to add and remove attachments.
+        /// </summary>
+        /// <remarks>
+        /// All add and remove attachment operations must synchronize on this for the lifetime of their operations.
+        /// </remarks>
+        Object AttachmentsSyncLock { get; }
+        
         /// <summary>
         /// The scene objects attached to this avatar.
         /// </summary>