Increase file reliability against group failure

[markberger]

Servers of happiness improves share distribution and guarantees a file can be recovered for up to n - h node failures. However, if a group of nodes fail, servers of happiness makes no guarantees. If I lose all the machines in my house, I have no way of knowing whether my other nodes have enough shares to reconstruct all my data.

One way of fixing this is to group a maximum of n - h nodes in a single location, but I think that solution is silly because I might not want to increase my n or lower my h to meet the requirement. Instead, I should be able to organize storage nodes into failure groups and guarantee that a subset of those groups will be able reconstruct every file. Given a set of groups with g cardinality, any subset with a cardinality of g - 1 must have k shares.

This is somewhat related to #467, but I think this ticket serves a different purpose.