compress Merkle DAG and origin nodes together

We've created a compressed representation of the archive Merkle DAG, from snapshot nodes down to contents. Strictly speaking that is exhaustive, but for some use cases (e.g., list all the different origins where a given node has been found) it would be useful to also have nodes representing origins. Now that we know compressing the full Merkle DAG is doable, we should give a try to the extended version that also includes origin nodes. It shouldn't be much harder, given the added nodes/edges are not that numerous.

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Migrated from T1867 (view on Phabricator)