o element.py / plugin.py: Removed supporting logging code, and derive
                            the log handle from Context.

  o _scheduler/scheduler.py, _scheduler/queues/queue.py: Adapt to new Job initializer API for the logfile

  o _scheduler/jobs/job.py: Run job activities within the new context manager
                            which turns on logging for a job. Also removed a lot
			    of custom logging abstract methods which are unneeded.

  o _scheduler/jobs: Job implementations need not implement so much custom logging.