Break up stager.py...

... into a new staging module with two additional submodules (up for setup_job and down for finish_job). Reorganize setup_job helpers in decreasing order of abtraction.
Refactor large function __download_results...

... into a class with smaller helper functions.
Restrict export from new staging.{up,down} modules.
Fix failing test (for when version file is not rewritten).