Fixes and improvements for T401

Summary:
Ref T401

• Fix : The 'ongoing' status wasn't set to the content that were supposed to be

in que archival queue.

• Refactor :
  • Extract sql query from archive worker and place it into db module
  • Refactor in order to improve code quality & separation and to simplify unit testing.
  • Change the way the director's configuration is stored to make it easy to send the whole config to ArchiverWorker

Reviewers: Reviewers

Maniphest Tasks: T401

Differential Revision: https://forge.softwareheritage.org/D27