upgrade to rufus-scheduler >= 3.0.0
tune testing and fancy printing
integrate @vjt 's concurrency test

originally in https://github.com/ifad/ruote/commit/a63f979cd9ece557dd6672f5849ab80f5634bb4b
fix https://github.com/ifad/ruote/commit/3a963899

(tested against the fs storage)
Merge git://github.com/jmettraux/ruote

* git://github.com/jmettraux/ruote:
  fix https://github.com/ifad/ruote/commit/3a963899
  integrate @vjt 's concurrency test
  tune testing and fancy printing
  upgrade to rufus-scheduler >= 3.0.0