[log improvement] no vacant CPU resources.
Created by: TomorrowIsAnOtherDay
Some of the xparl users reported that they have K CPUs in the cluster, but xparl will log:
no vacant CPUs, when the task requires K CPUs
repeatedly in a short time.
Generally, users are encouraged not to create and destroy actors repeatedly. Maybe we should tell users this problem if he/she creates and destroys the actor repeatedly.