Support minimum replication level for snapshots

[sfc-gh-tclinkenbeard]

Running with triple replication, we may run into a situation where a single storage server cannot be snapshotted for whatever reason. This will currently cause all snapshots to fail. However, it might be tolerable to let the snapshot succeed without this one snapshotted disk, with the knowledge that when we restore from the snapshot, the data distributor will have to restore full replication. We can extend the snapshot API to support a minimum replication level argument, so snapshots still succeed as long as they are able to snapshot enough replicas of the data.