Fix race between reading file info and bitcask merge delete
Fix race between reading file info and bitcask merge delete

Between listing a directory and reading each file infos, merge
delete can interleave. Then read file info results in enoent error.
This commit treats it as zero size file.

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