GitLab Migration - Make it possible to parallelize pipelines
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## The problem
Some pipelines might be too time and memory consuming. Imagine a pipeline that batch loads multiple snippets, then for each snippet we clone their repository. If a project has thousands of snippets, this might mean thousands of requests to clone repos and a very long time that this job would be taking from one of the Sidekiq job workers, basically taking this worker all for itself.
## The proposal
Work on the pipelines so that we can make them splittable through Sidekiq workers. This will guarantee a better distribution of the Sidekiq workers and will also allow pipelines to be executed faster, as they'll be parallelized.
/cc @acook.gitlab @georgekoltsov @hdelalic @kassio @dstull
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