accumulate multi-keep mapred query outputs more efficiently
accumulate multi-keep mapred query outputs more efficiently

as with single-keep, avoid repeated list appending, by building the
result list backward with cons

esl/riak-erlang-client,paulperegud/riak-erlang-client,GabrielNicolasAvellaneda/riak-erlang-client,SiftLogic/riak-erlang-client,groovenauts-erlang/riak-erlang-client,inaka/riak-erlang-client,linearregression/riak-erlang-client,rhumbertgz/riak-erlang-client,jmayfield74/riak-erlang-client,pma/riak-erlang-client