Reap lookup tables in DeleteUnretainedHistory

[tamirms]

Update the reaping goroutine so that it deletes rows in the history lookup tables which are outside the retention window. This should be as simple as executing a DELETE statement like:

DELETE FROM history_accounts WHERE latest_referenced_ledger < $ledger_boundary;

For more context, see: Improving Reap Performance of History Lookup Tables

[tamirms]

We decided to go with a different design: https://docs.google.com/document/d/1CGfBCS99MTEZDP4mMhV1o6Z5NE_Tlg7ENCcWTwzhlio/edit