This PR implements the missing functionality for archive nodes by
pruning stale index data.
The current mechanism is relatively simple but sufficient for now:
it periodically iterates over index entries and deletes outdated data
on a per-block basis.
The pruning process is triggered every 90,000 new blocks (approximately
every 12 days), and the iteration typically takes ~30 minutes on a
mainnet node.
This mechanism is only applied with `gcmode=archive` enabled, having
no impact on normal full node.
