Figure out need_to_collect function in entity plugin

[johannaengland]

nav/python/nav/ipdevpoll/plugins/entity.py

Lines 62 to 68 in d79c121

	@defer.inlineCallbacks
	def _need_to_collect(self):
	yield self.stampcheck.load()
	yield self.stampcheck.collect([self.entitymib.get_last_change_time()])
	result = yield self.stampcheck.is_changed()
	defer.returnValue(result)

Originally posted by @lunkwill42 in #3029 (comment)

This function has been unused since being introduced and it should be looked at to either remove it or improve it in a way that it can be used.

[lunkwill42]

This function has been unused since being introduced and it should be looked at to either remove it or improve it in a way that it can be used.

That assessment is incorrect. The function was in use when it was introduced, but the call to it was commented out in a later release, due to collection problems that arose. IIRC, some devices would not update their timestamps properly, even if the entPhysicalTable had been updated, and this would cause the plugin to never retrieve any of those updates, instead opting for its cached version.