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[irevoire]

Fixes #117

• Since we’re not breaking random descendant, the frozen reader only contains the useful number of items that fits in RAM instead of containing all items

TODO:

• When we insert the items in the tree, we must retrieve all the descendants that became too big
• We should also add to this list of descendant the list of new made tree node that contains the whole DB.
• Once this is done, we can start the loop over this whole list and treat each item's one by one
• Then test and fix
  • We should remove descendants when they became too small and check again the reuse_node_id test
  • When remove an item that was a branch of a split node we don’t rewrite the node and don’t remove the item
• Then parallelize