DB storage : storing hashes along with the commitment

[thomas-quadratic]

Currently, commitments' scalars are stored inside the children nodes in memory and in database.
However, for generating proofs, we need to get all scalars either way.
We propose to store them along the commitment in the database at the parent node.

To minimize refactoring for now, the idea would be for a StoredNode's NodeFactory to retrieve the parent node info + children hashes from DB, create the parent node, and distribute the children hashes to the children.
This way, a proof generation will not need to load the children node only to retrieve hashes.