Improvement concerning the release of resources

[giffels]

It seems to be reasonable to take into account also the age of resources, when releasing them. Currently, only the utilization and supply is used to decide, if a resource well-suited to be released next.

Background to this feature request can be found in MatterMiners/tardis#172.

[maxfischer2781]

For reference, this is about how the FactoryPool selects the children to reap. Current code:

cobald/src/cobald/composite/factory.py

Lines 99 to 104 in e9828eb

	def _shrink(self, target: float):
	# we can only reap children that are not already shutting down
	# prefer reaping children that supply few used resources
	hit_list = sorted(
	self._hatchery, key=lambda child: child.supply * child.utilisation
	)