check_snapshot: Check existence down to contents
This now checks from the top to bottom that all objects referenced by a snapshot exist.
I'm opening the diff as a correctness first approach. Did not refactor the common code yet.
Also i believe i'm missing the nested directory check scenario (but that's not really a blocker).
Related to T2483 Depends on !390 (closed)
Test Plan
tox
Migrated from D3503 (view on Phabricator)