Monitor for AssetBlobs with undefined sha256

[dchiquito]

The sha256 checksum of newly uploaded blobs is calculated by an asynchronous task, which may fail. We should monitor the DB for older AssetBlobs which do not have a sh256 field, and have a way to conveniently recompute those checksums as necessary.

#254 is an instance of this happening.

[yarikoptic]

FWIW, I have reverted not issuing a warning in our dataladification script: dandi/dandisets@3cd51b5 so now I think I should be getting those warnings in cron job report. Not a solution, but at least we might have some means (if I spot it) for detecting if smth like that happens again.

edit: note that the check would happen only if assets list (with metadata) got any changes.

so I tested if all assets currently as after a fix which fetched all updated and now have sha2-256 have it - and it seems to be all good

dandi@drogon:/mnt/backup/dandi/dandisets$ for d in 000*; do jq '.[].metadata.digest."dandi:sha2-256"' $d/.dandi/assets.json | grep -q '^null' && echo 'missing'; done
dandi@drogon:/mnt/backup/dandi/dandisets$