Currently batched background migrations are executed sequentially, in _first in, first out_ order. Since we are now extensively using only batched migrations we regularly have a queue of migrations waiting to be executed. Given that these migration usually take some time (hours or days), there can be quite a long delay from the time migration is enqueued till the the time it is completed. ### Proposal Parallelize execution of batched background migrations. This should meet the following criteria: - [x] Execute up to `X` migrations simultaneously. Ideally `X` should be configurable. - [x] Do not execute migrations that target the same table at the same . - [x] Do not execute jobs from the same migration at the same time. <!-- Label reminders Use the following resources to find the appropriate labels: - https://gitlab.com/gitlab-org/gitlab/-/labels - https://about.gitlab.com/handbook/product/categories/features/ -->