Add multi-DB support to loose foreign keys

How:

1. Iterate over the connections within the CleanupWorker class.
2. For each connection invoke the CleanupService
3. Within the CleanupService, re-connect the LooseForeignKeys::DeletedRecords model to the given connection.

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The following discussion from !69165 (merged) should be addressed:

• @ayufan started a discussion: (+2 comments)

  Technically this worker does not work with multiple databases, but I assume this is next step.