ProtobufZOFits keeps all messages in memory until dtor

See lstchain issue below: https://github.com/cta-observatory/cta-lstchain/issues/1280

Basically we do a data volume reduction (pixel selection) for a subrun, and it is done sequentially for each of the 4 streams it is composed of. The memory the process takes keeps growing as it finishes one stream and goes on to process the next. Is there a way to free the memory after each stream?