Scheduling: Waiting for completion of previous task

[tinyoverflow]

Is your feature request related to a problem? Please describe.
I'm creating backups of my servers with the backup system every morning. Unfortunately, the current implementation of schedules doesn't allow for data consistency.

Describe the solution you'd like
As a panel client, I would like to be able to decide to either delay a task by some amount of seconds, or to wait until the previous task completes or gets to a predefined state. This is especially useful for data consistency in backups as it would allow to stop the server completely, only then start the backup and start the server again only when the backup was completed.

I said "or gets to a predefined state" earlier. With that I specifically mean backups: When using the S3 adapter, uploading can take some time, depending on various things. The server can be started once the backup file has been created. It doesn't need to be offline for the uploading process, therefore waiting for completion results in unecessary downtime.

Describe alternatives you've considered
I've considered using manual delays between the tasks. While this works, it doesn't scale well and it has to be adjusted everytime the server grows. Another option would be to just use big delays from the beginning, but that would mean an unecessary downtime.

[DaneEveritt]

This is a bit more than an enhancement @schrej (at least in terms of work). I foresee a lot of complexity with signaling between Wings and the Panel for a lot of these.

[DaneEveritt]

Duplicate of #2222 favoring this since you have an account still.

[jack3898]

Hi Dane, is this a WIP?

[aglarcz]

Any news on this?

[gdias1992]

Any news on this?