store: Create postponed indexes on subgraph restart

Replace the IndexList-based `recreate_invalid_indexes` call in
`start_subgraph()` with a call to `create_postponed_indexes()`. This
uses `IF NOT EXISTS` and `CONCURRENTLY` to safely create any missing
postponed indexes on every restart, acting as a safety net.

Remove the now-unused `IndexList::recreate_invalid_indexes` method.