Can delete the Offline Messages sent to/from a user.

This is useful if the user is deleted.

OpenSim/Services/Interfaces/IOfflineIMService.cs

@@ -35,7 +35,14 @@ namespace OpenSim.Services.Interfaces
     public interface IOfflineIMService
     {
         List<GridInstantMessage> GetMessages(UUID principalID);
+        
         bool StoreMessage(GridInstantMessage im, out string reason);
+        
+        /// <summary>
+        /// Delete messages to or from this user (or group).
+        /// </summary>
+        /// <param name="userID">A user or group ID</param>
+        void DeleteMessages(UUID userID);
     }
 
     public class OfflineIMDataUtils