Measure the gas costs of processScheduledPieceMetadataRemovals for different numbers of deletions

We do `processScheduledPieceMetadataRemovals` in `nextProvingPeriod`, what's the limit to that and can you schedule enough deletions in a proving period to prevent `nextProvingPeriod` from being called? We should know the limit and protect against this because it could make it impossible to progress with proving a data set.