Allow multiple storage spaces when replicating

When replicating, we will soon approach a point where we will need to select from multiple preservation areas to replicate into. At the moment it makes sense to choose a single space per replication. Whether we will want to distribute data randomly (rendezvous hashing), or some other type of scheme (distribute on disk usage) is tbd.

Before we can start work on this, #32 (closed) will need to be completed.