Created an creation object for ArchiveToFileRequest.

This simplifies to management of the 3 step operation in the DB:
- Create a request in the scheduler DB
- Create the coresponding files in the name server
- Mark the request as good to go and queue it in the tape pools, or
  delete it.

In addition, the destructor of the creation object allows early
garbage collection in case operation was not completed correctly
(failure to call complete() or cancel())