Change the format of the archive file by saving the relative path of the directories
and files to be transfered. This simplifies the path merging for both migrating and
staging.

Save the archive file directly in EOS at the root directory to be migrated. The name of
the archive file is by default archive.json.

Change the way a mover process uses the archive file. It first downloads it to the local
disk and then uses it to do the transfers while at the same time saving a log of the
current transfer. At the end of the transfer or when an error occures the log file is
copied back to EOS in the same location as the archive file.
Clean all files used by the tracker in the event of a failure or at the end of a
successful transfer.

Restructure the code by using just one daemon process with subprocesses for migration
or staging.