Resolve "Review siafile package for correct use of wal update methods"

Closes #3422 (closed)

I changed a few occurences of SaveHeaderUpdates to SaveMetadataUpdates. SaveHeaderUpdates only needs to be called if the pubKeyTable changes. Otherwise SaveMetadataUpdates is sufficient for simple changes to fields of staticMetadata. If the change causes the serialized metadata to be too large, SaveMetadataUpdates will call SaveHeaderUpdates itself.