Implement `data_size` in multibackend
Implement `data_size` in multibackend

Port of c582cadccd72208720b0d0f1379e56e2a1119c82 from riak_kv

GabrielNicolasAvellaneda/riak_cs,dragonfax/riak_cs,laurenrother/riak_cs,dragonfax/riak_cs,yangchengjian/riak_cs,sdebnath/riak_cs,basho/riak_cs,yangchengjian/riak_cs,basho/riak_cs,basho/riak_cs,sdebnath/riak_cs,yangchengjian/riak_cs,sdebnath/riak_cs,dragonfax/riak_cs,yangchengjian/riak_cs,GabrielNicolasAvellaneda/riak_cs,dragonfax/riak_cs,laurenrother/riak_cs,GabrielNicolasAvellaneda/riak_cs,GabrielNicolasAvellaneda/riak_cs,sdebnath/riak_cs,basho/riak_cs,laurenrother/riak_cs,laurenrother/riak_cs