If "fcache assets" is executed, don't stop with an exception if there's an access violation if another thread is operating on the file at the same time.

Resolves one issue from http://opensimulator.org/mantis/view.php?id=7271

OpenSim/Region/CoreModules/Asset/FlotsamAssetCache.cs

@@ -795,7 +795,7 @@ namespace OpenSim.Region.CoreModules.Asset
 
                         if (File.Exists(filename))
                         {
-                            File.SetLastAccessTime(filename, DateTime.Now);
+                            UpdateFileLastAccessTime(filename);
                         }
                         else if (storeUncached)
                         {
@@ -811,7 +811,6 @@ namespace OpenSim.Region.CoreModules.Asset
                 });
             }
 
-
             return uniqueUuids.Count;
         }