Conduit: re-indexing

[winder]

Followup to #1243

Add a configuration for "next round override". This would override the round, if any, found in the pipeline metadata file.

Importers would be able to detect that the round has moved forward/backwards during initialization and initialize themselves accordingly.

This feature is part of:

• Re-indexing: i.e. if there was a plugin data bug on round 123456, you could set the round to 123455 and re-import the data to repair the corruption.
• Skipping rounds: i.e. if you only care about recent data, you can skip the old data.

The importer / local ledger plugins can handle this situation automatically by automatically running a fast-catchup that loads an appropriate catchpoint for the desired round.