Cancelling stable-rc jobs when a new -rc is produced

Currently if a new stable-rc is produced then we allow any currently pending build and test jobs for a currently existing -rc are left queued. Given the volume of jobs stables produce it's likely that there will be a bunch of these and since the stable trees are rebuilt from patch queues the old test results will no longer be relevant it would be beneficial to detect this happening and cancel any pending work.

This could be perhaps be implemented more generally by cancelling jobs for a tree f the commit being tested vanishes from the history of the branch being monitored.

No files or tests are named. Start by locating the stable-rc job scheduling and branch or commit monitoring entry points; the work is complete when pending jobs for an older stable-rc are canceled after a newer stable-rc is produced, without leaving obsolete work queued.

Written by the indexing model from the issue text.