[v6r13] Clean-up and changes in the Storage classes

These are a number of changes in order to simplify the logic of the Storage classes:

    - single place where the PFNs are constructed from LFNs ( StorageBase.getPfn() )
    - multiple methods from specific Storage classes are moved to StorageBase
    - StorageFactory creates Storage objects using a standard plugin mechanism ( ObjectLoader )
    - check if StorageElement is allowed for the user's VO
    - dirac-dms-show-se-status shows only SEs eligible for the user's VO
    - fixes in the TestXXX classes
    - suppressed ReplicaManager, MetadataCatalogClient
    - some bug fixes and minor improvements Those changes are still to be thoroughly tested.
Where is this getPfn defined ? And what is getPfn anyway ?