Add a fast and reliable way to check history

[2opremio]

We should have a fast and reliable way to know what has been ingested and if there are any problems (e.g. gaps).

The new transactions DB backend provides GetLedgerRange, using the metadata sequences, but we should at least index that field (I wonder what the performance is in current code :S).

The method should be thorough and have some (at least minimal) consistency checks to detect things like gaps, partially ingested data etc ...