Fix instance idle time not resetting after use
When scaling down, we check which instances can be removed. A bug in the implementation was removing instances too early, because even if they had just been used, their idle wasn't being reset.
This MR introduces a last used time for the instance, rather than solely using their provisioned time.
First part of gitlab-org/gitlab-runner#30953 (closed)
Edited by Tomasz Maczukin