Implement synchronous history backfilling

[2opremio]

As part of #196 , the wanted history window may be smaller than the existing window.

As part of this task we should create an offline ingestor (e.g. not real time) which backfills range of ledgers. The orchestration of what ledgers to ingest should be part of #202

The ingestor should (together with the orchestrator) to make sure no inconsistencies are left on the database.

In particular, it should ingest ledgers backwards, to ensure that no gaps are left on the database:

[2opremio]

Reopening as part of the backfill history

[mollykarcher]

See #323