Refactor/Unify cross region looping

[sopel]

This is basically a cross cutting technical debt and thus dependency of both #32 and #41:

• The cross region loops are currently implemented via procedural code and should be refactored to a functional design pattern in order to remove respective code duplication and improve separation of concerns.
• Ideally this should also abstract the legacy S3 region handling difference, which is complicating region related code for S3 for no apparent reason in general and within this project in particular.