dashboard/app: periodically remove coverage garbage from spanner

[tarasmadan]

No description provided.

[tarasmadan]

https://cloud.google.com/spanner/docs/foreign-keys/overview#how-to-define-foreign-key-action may be a better way to go.
Spanner allows to auto-delete referenced data.
Potential problem - what if the referenced data requires >80k mutations?
Currently 1 "merge_history" record points to up to 15k "files" records. It requires min 15k mutations to delete it. Index mutations are also counted somehow (2-3-4x in the worst case for 1-2-3 indexes).
Positive side - the 80k limit was introduced last year. It was 40k before.

P.S. It looks good for the smaller operations where you can fit everything in a single transaction.
In our case it is hard to say whether it makes sense to change existing code.
There is a high probability merge_history session change will trigger the the cascaded deletion of more that 80k rows (file records, indexed and potentially syz-lang programs int the future).

[tarasmadan]

ptal

[tarasmadan]

PTAL

[ramosian-glider]

LGTM in general