Add a QueryItem method to the inventory subsystem. Currently implemented for

MySQL only, stubs for the others. This allows updating the cache with a single
item from the database.

OpenSim/Tests/Common/Mock/TestInventoryDataPlugin.cs

@@ -115,6 +115,11 @@ namespace OpenSim.Tests.Common.Mock
         public void updateInventoryItem(InventoryItemBase item) {}
         public void deleteInventoryItem(UUID item) {}
 
+        public InventoryItemBase queryInventoryItem(UUID item)
+        {
+            return null;
+        }
+
         public void addInventoryFolder(InventoryFolderBase folder)
         {
             m_folders[folder.ID] = folder;