Dust Expiry: Clean the UTXO set from spam

[gmaxwell]

@ajtowns

The proof costing weight is a disincentive to spending dust, but not to creating it, though, so mostly makes this worse, rather than better, as far as I can see?

No effect to dust as a semaphore, I think. Disincentivizes creating many tiny outputs when you could create fewer larger ones, under the assumption they’re actually going to spend them eventually and not just use them for sequencing.