cache: investigate and possibly disable caching

[mdonadoni]

Caching has caused an issue recently (see #420) even though it is not enabled in REANA. In particular, the config variable CACHE_ENABLED is used in reana-workflow-engine-serial to disable the caching mechanism, but it seems like this is not enough as there is more caching logic in reana-job-controller that is always executed for every workflow.

We should investigate what this logic is doing and whether it's safe to disable it using the same CACHE_ENABLED flag. Note that this should improve performance, as currently we are saving lots of file metadata to the database, even though they are then not used:

reana-job-controller/reana_job_controller/job_manager.py

Lines 127 to 137 in e6742f2

	def cache_job(self):
	"""Cache a job."""
	workflow = (
	Session.query(Workflow).filter_by(id_=self.workflow_uuid).one_or_none()
	)
	access_times = calculate_file_access_time(workflow.workspace_path)
	prepared_job_cache = JobCache()
	prepared_job_cache.job_id = self.job_id
	prepared_job_cache.access_times = access_times
	Session.add(prepared_job_cache)
	Session.commit()

[mdonadoni]

Caching in reana-job-controller has been disabled as it was already disabled in reana-workflow-engine-serial and to improve performance (see the graphs in #435)