[backend] Code re-organization into the object model

This commit brings the following changes:

* Rename Job.KILL by Job.CANCEL as we normally send SIGTERM instead of SIGKILL
* Move most of code related to JobSplit/Worker back into the object model, from
  the schedule module
* Implement signal handling on scripts that will run under the supervisord
* Overall clean-up of backend standalone scripts, moved code back into the
  object model
* Rename standalone scripts (remove '.py' extension)
* Reflections of these changes to other code