Stop pointing to item directly in split nodes

[irevoire]

Currently, when crafting split nodes, arroy may output a direct link to an item on one of its sides.
That’s an optimization that removes one indirection:

• But it practically never happens in real life
• /!\ It stops us from multi-threading some parts of the code

We should update the split node creation to include only other tree nodes (descendants or other split nodes).
This is a DB breaking, which means:

• We have to provide an upgrade path
• It’s the perfect occasion also to implement Make the normal in the splitnode null instead of a vector of 0s #55
• We must still be able to read the previous version of the database