backfill: use the common `storage` top-level config key
This makes the backfiller configuration compatible with all other modules.
Depends on !594 (closed)
Test Plan
tox tests updated
Migrated from D4210 (view on Phabricator)
This makes the backfiller configuration compatible with all other modules.
Depends on !594 (closed)
tox tests updated
Migrated from D4210 (view on Phabricator)