Implements support for blocked evaluations on system jobs

[maihde]

Fixes #4072

[preetapan]

@maihde We are going to refactor the system scheduler to support deployments and blocked evaluations to make it closer to the generic scheduler. Will definitely use parts of your commit in our work. I am going to close this specific PR due to age and merge conflicts, its easier to start afresh since the scheduler has changed significantly due to preemption/affinities recently.
We definitely appreciate your efforts, sorry we couldn't get to this sooner.

[github-actions]

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