Add a kvlayer backend based on Riak (http://basho.com/riak/).

This makes heavy use of Riak secondary indexes for scan(), and so Riak
must be configured to use eLevelDB as its backend.  The backend defaults
to the much-faster protobuf API, over the slower-but-readable HTTP API.
Set storage_type: riak, and set *multiple* storage_addresses for the
Riak cluster.