[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
All up for that, and it's already being done :-)