Conduit: Save round metadata at pipeline level.

[winder]

The pattern where the round is stored in the exporter prevents allowing multiple exporters to be used, and requires all exporters to maintain the latest round regardless of whether or not it adds value. It also means features like "re-indexing" would have to be implemented at the exporter level, which seems like a roundabout way of configuring things.

Proposal

Conduit should maintain a metadata.json file and initialize itself without the help from the exporter.

In the future we may want to add a flag that overrides the current round. Another name for the override would be "re-indexing" / "replaying" / "initialize at round".

Acceptance criteria

Take the metadata.json file implementation from file_exporter.go and put it in the pipeline.

Simplify the interfaces, it should no longer be necessary for the exporter to report which round it is on.