Direct CAS-to-CAS import

Description

Add direct CAS-to-CAS import in CasBasedDirectory.

Proposed changes

• Rework of CasBasedDirectory.list_relative_paths so it gives results in the same order as the list_relative_paths in utils
• Import files directly instead of using a filing system round trip
• Unit test which compares the results with the file system round trip
• Rework CasBasedDirectory.list_modified_paths so it gives results in the same order as a filtered list_relative_paths
• Performance test

(Task or bug number that this MR solves preceded by #)

This merge request, when approved, will close: #574 (closed)

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