New strategy for document validation

[davidbrochart]

Problem

The current method for validating documents is having a copy of the document, applying changes to it, and if it's still a valid document, applying the changes to the original document. It's expensive because documents and operations are duplicated.

Proposed Solution

If updates are stored e.g. in a YStore, a better solution could be to always apply changes to the original document, and if it fails validation, create a new document from the stored updates (and not store the last update).

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