backfill: Dedicate storage configuration for the backfiller
Prior to this commit, we would share the storage configuration for both storage and the backfiller. This would end up having possibly a journal_writer configuration key on some instances.
This reworks the configuration to allow a read-only storage configuration. And installs the required logging configuration file.
Related to T3009
Test Plan
bin/octocatalog-diff is happy
Migrated from D5058 (view on Phabricator)