Improve XForm.last_submission_time

[jnm]

kobocat/onadata/apps/logger/models/instance.py

Lines 80 to 83 in f1302f9

	XForm.objects.filter(pk=instance.xform_id).update(
	num_of_submissions=F('num_of_submissions') + 1,
	last_submission_time=instance.date_created,
	)

last_submission_time should probably be set to the maximum of whatever's already in the database or instance.date_created, not instance.date_created unconditionally. The F expression for num_of_submissions takes away race conditions there, but for last_submission_time, there's a chance of us overwriting a newer timestamp with an older one.