FR: implement 'disperse 2n redundancy n' volumes

[freopen]

I think disperse 2n redundancy n has its cases. For example, imagine we have two servers, each with four HDDs of the same size. You would want to store full dataset on each server to be able to read when one of the servers is offline. Currently the only option to do that is to have replica 2 volume, but that means you risk data loss after losing just two drives. If disperse 8 redundancy 4 was a thing - the data loss risk would appear only after 5 lost drives. I realize that the latency and the throughput will suffer, but I think it's a viable tradeoff for cold data.