FIX: Replication of Repositories

This failed because of two things:
* `cvmfs_server add-replica` generated a misconfigured *upstream string*
* `BackendStat` was dead code that needed to be replaced but wasn't yet.

**Note:**
The temporary directory (download location for replicated files) might not work with Riak at the moment. This is kind of tailored for local backends in `cvmfs_server add-replica/snapshot` and might be replaced by a flexible C++ implementation to take different backends (with possibly different replication techniques) into account.