[Direct Transfer] Do not push children files in the queue at first

When creating a new DT session with folders, all folders and files will be added to the queue. The queue will process all items first, and then children of the top-level folder will be repushed because their parent is not yet uploaded.

We should improve that logic:

• Either by pushing in the queue only the top-level folder and, when processed, pushing its children.
• Either by rethinking how hierarchy are uploaded (should we handle folders first? and then all files?).

It is open to suggestions.