Limit max concurrent jobs

[Kidswiss]

The operator should have the ability to limit how many jobs should get run concurrently.

This limit should be configurable by job type (prune, backup, check, etc.)

[Schnitzel]

additonal thing: run only one backup/prune/check per repo/s3-bucket.
As I don't think you can backup to the same repo/s3-bucket at the same time?

[Kidswiss]

@Schnitzel concurrent backups to the same repository are supported by restic. Although the deduplication may not be optimal in that case: restic/restic#376

However I observed some issues with restic's own locking when the backups are triggered at exactly the same second.

[susana-garcia]

I'll take this one