Instead of storing the (tree) data directly at the root of the
object specific directory, move it into a `data/tree` subfolder.
This prepares for two things:
1) the `tree` folder will allow us to add another folder next to
it to store metadata.
2) storing both, `tree` and the future metadata folder in a
common subfolder, prepares for the future integration
with the new caching layer (`FsCache`).
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