[v6r13] refactor dms

This PR contains most of the changes for the long awaited DMS refactoring.
Now, the DataManager and the StorageElement will accept only LFN as input. The concept of PFN should not exist anymore anywhere outside of the Storage plugins.
The code still contains various mentions of the PFN, which are stored in local db or so, but they are not used in practice, and should be cleaned up as well.

A few items still need to be addapted : The Sandbox system as well as the StorageElementProxy handler will be taken care of by @atsareg. Two of the methods of the API (getPhysicalFileAccessURL/getPhysicalFileMetadata) need to be removed or adapted, and this will be taken care of by @phicharp in a broader adaptation in sync with Ganga.

All the test classes need to be adapted. This is an early (if not premature...) PR to be used as a base for dev on v6r13. Only the DMS actual functionnlities have been (successfully!) tried, such as replication.