[v6r12] review existing dfc

This PR contains only fixes, improvements, docs and so on for existing DFC code. 
Changes should be transparent except for 1 commit (https://github.com/chaen/DIRAC/commit/de7d410ae57f94e053e3904db6b2e5e80e2112e4) This brings some changes to the DB in order to improve performance:
* all tables are innodb
* smallint replaced with int in the FC_DirectoryLevelTree table

The change to innodb is mandatory to avoid the table locking

      I see that you have reviewed all the
      uses of statuses, very good. I suggest that
      in the code we always use statusID for numeric status and just
      "status" or "statusString"
      for the string status. This is a DIRAC style.
        Cheers,
        Andrei
      On 17/06/14 09:28, chaen wrote:
    
      In
        DataManagementSystem/DB/FileCatalogComponents/FileManager.py:
      >      connection = self._getConnection(connection)
>      if type(fileID) not in [TupleType,ListType]:
>        fileID = [fileID]
>        
> -    if paramName == 'Status' and not paramValue in self.db.validFileStatus:

      What a careful review :-)
        Because the paramValue is the id of the status, not the name
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