Optimise dag processing

What does this MR do?

This makes DAG processing way cheaper, as we only process jobs that are guaranteed candidates to be complete.

We tend to find all jobs with DAG, that are:

1. Dependent on jobs triggering processing,
2. Do not have dependencies that are not finished.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/65538

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/65457