[feature] Add a "repair" mode

[zivkovicmilos]

Description

Add an indexer repair subcommand that would verify the integrity of all blocks / transaction results, and fetch the ones that are missing (with retries).

In the current indexer implementation, I have opted to ignore blocks or transactions that are un-fetchable (for one reason or another, usually backwards compatibility support), so this subcommand should verify the DB is up to date with the chain data.