Fixes and improvements for T401

Ref #401

- 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

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Migrated from D27 (view on Phabricator)