Data lake changes

Change in the way wendelin handles split file ingestions. The merge of datastreams (corresponding to the file chunks) is no longer done in the server side. Several scripts were adapted accordingly.

This change has impact on the client side ebulk tool: https://lab.nexedi.com/nexedi/ebulk/merge_requests/1/

This MR is a "clone" of previous https://lab.nexedi.com/nexedi/wendelin/merge_requests/40/ due to an issue in the automated tests that required to create a new branch. That MR contains more discussion about these changes.

Passing test results: https://nexedijs.erp5.net/#/test_result_module/20200618-B3C60D56