Set maximum lifetime for a GitHub runner

[bbockelm]

The service node should initiate the shutdown of idle GitHub runners that have already run for some pre-configured threshold (suggestion: 4 hours).

By turning over these runners, we will ensure others get the opportunity to grab the slots in the pool.

Notes:

• Only remove runners that are idle. Don't worry about race conditions: requests to remove a runner that just picked up a new action are ignored.
• Only way to gracefully shutdown runners is by removing the runner via the GitHub API. If Remove VMs that fail to connect to GitHub #3 is implemented, we should set an attribute in the job ad that states when the graceful shutdown was initiated to avoid the cleanup script initiating a hard-shutdown of a VM that's doing a graceful shutdown.