LArRecConditions+LArCalibTools+LArRecUtils: Clean up LArFebConfig.

Remove mutable cache in LArFebConfig --- it can't really be making any significant difference. But add a method to retrieve both upper and lower thresholds in one go. Thus can also remove the locking. Convert the map to an unordered_map. Add a method to fill the map, allowing the removal of the friend declaration.