Scheduler should not create Backups until outstanding backups for the schedule have completed

[dsu-igeek]

What steps did you take and what happened:
If a scheduled backup time is reached while a previous backup for the schedule is running, a new Backup resource is created. This can result in multiple Backups being queued that will all get executed together when the queue is unstalled.

What did you expect to happen:
Backups should not be queued until any previous backups for the schedule have completed.

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[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

Closing the stale issue.

[reasonerjt]

Not a stale reopening.

[blackpiglet]

This issue is already fixed by PR #5283