Add back postcard fingerprints.csv

[sffc]

Add back postcard fingerprints.csv

[sffc]

It would be nice for the DataExporter to return the stats somehow. Two ideas, neither of which is great:

1. Return pub struct PayloadStats { pub digest, pub size } from put_payload(). This works fine in BlobExporter, but in BlobExporter, we don't actually serialize the payload to a TokenStream until the flush function.
2. Return pub struct DataExporterStats { pub marker_identifier_to_payload_stats: HashMap<...> } from close(). This is a bit of a pain to calculate in BlobExporter because we've already lost most of that information.

Maybe we could keep the current model of StatisticsExporter, which handles payloads just fine, but add in a way to return the lookup table sizes from the DataExporter. This would need to be in the close() function because BlobExporter doesn't make the lookup tables until the very end.