Group Migration - Concurrent model

Currently as part of the MVC a new migration is done inside of a singular background worker !44836 (merged)

However, as we scale and add support to import complex group and project structures, a single worker will no longer suffice.

Add an ability to perform import process in a distributed fashion.

Potential solutions:

1. Use Gitlab::JobWaiter to perform distributed import. A draft implementation can be found here 0a74b7bb Job waiters are used in GitHub and Jira Parallel Imports
2. ...