Switch from tombstoning deleted objects to actually deleting them

[snarfed]

When we get a Delete, right now we set Object.deleted to True, and we have to check that flag everywhere. This originally seemed like a good idea, long ago when I was young and foolish, but I don't know that it's ever bought us anything, and it's a liability. We should switch to actually deleting.

[snarfed]

One catch: we can't delete Objects in Protocol.receive because send tasks need their copies. So we'd need to delay that delete, maybe with Object._expire.

[snarfed]

^ Done, via Object._expire.

I'm also running a workflow to delete all existing deleted Objects right now. 18M! Using https://cloud.google.com/dataflow/docs/guides/templates/provided/firestore-bulk-delete#gcloud , based on https://github.com/snarfed/bridgy#delete-old-responses :

gcloud dataflow jobs run 'Delete Object.deleted datastore entities' \
  --gcs-location gs://dataflow-templates-us-central1/latest/Firestore_to_Firestore_Delete \
  --region us-central1 \
  --parameters firestoreReadGqlQuery="SELECT __key__ FROM Object WHERE deleted = TRUE",firestoreReadProjectId=bridgy-federated,firestoreDeleteProjectId=bridgy-federated,firestoreHintNumWorkers=100

[snarfed]

Next step, delete all old Objects (eg >90d) that should now be expired due to our new logic.