Arbitrary slicing on PathSlicingObjStorage
Allow the sha1 slicing of a content to be fully customizable.
For example, a content's sha1 : "abcdef1234567890" in a storage with slicing "0:2/0:5" will be stored at "root/ab/abcde/".
Also, make the required changes in the swh.storage package to follow those modifications.
Migrated from D49 (view on Phabricator)