implement list_bucket for gb_trees backend
implement list_bucket for gb_trees backend

wwezhuimeng/riak_search,authbox-lib/riak,xuejianhai/riak,GabrielNicolasAvellaneda/riak,DavidAlphaFox/riak,yangchengjian/riak,planobe/riak,xuejianhai/riak_search,xuejianhai/riak_search,elkingtonmcb/riak,glickbot/riak,laurenrother/riak,basho/riak_search,planobe/riak,amyvmiwei/riak,basho/riak_search,wwezhuimeng/riak_search,basho/riak,biddyweb/riak