PluginInstance start_date semantics

[jennydaman]

As a hotfix to #408, we now have the code:

ChRIS_ultron_backEnd/chris_backend/plugininstances/services/manager.py

Lines 194 to 197 in a1a2589

	# https://github.com/FNNDSC/ChRIS_ultron_backEnd/issues/408
	now = timezone.now()
	self.c_plugin_inst.start_date = now
	self.c_plugin_inst.end_date = now

This solution isn't ideal, because it means start_date and end_date have bogus values up until the status finishedSuccessfully, finishedWithError, or cancelled is achieved.

ChRIS_ultron_backEnd/chris_backend/plugininstances/models.py

Lines 26 to 33 in a1a2589

	STATUS_CHOICES = [("created", "Default initial"),
	("waiting", "Waiting to be scheduled"),
	("scheduled", "Scheduled on worker"),
	("started", "Started on compute env"),
	("registeringFiles", "Registering output files"),
	("finishedSuccessfully", "Finished successfully"),
	("finishedWithError", "Finished with error"),
	("cancelled", "Cancelled")]

It would be ideal if there was a timestamp recording the time of each of these state changes: created, waiting, scheduled, started, registeringFiles, and "finished" (one of finishedSuccessfully, finishedWithError, or cancelled).