Group Migration: Spike on concurrency Models

Problem

With the complexity of Group Migration increasing with more relations/associations being imported, a concurrent model becomes required to

Reduce the memory consumption on smaller processes
Provide better user, and developer, experience by having more granular status about the process
Improve the Migration process resilience, since retrying a small parts of the migration is more feasible than the whole migration
Provide an easy way to handle Network Rate Limits (related to retry smaller parts of the migration)

Spike on a first iteration of a concurrency model to be added to the Group Migration (BulkImports) to unblock the benefits above.