logilab-database #291591 backup_commands() for sqlite should return a list of lists [in-progress]

During a db-dump in CubicWeb, backup_to_file iterates on cmd as returned by DB helper's backup_commands method. Then it passes this to _run_command which does a ' '.join(cmd). So, obviously, cmd should be a list of strings, not a string.

From 37d856bfd6f3, backup_commands() returns a string instead of a list of string. So db-dump is unsuable for sqlite backend.

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patchLet {backup,restore}_commands methods return a list of lists for sqlite backend [pending-review]