Avoid expensive orrdict:append in MapReduce result consumption

This is the same improvement as was made in these commits for the protocol buffers client:

 - https://github.com/basho/riak-erlang-client/pull/58
 - https://github.com/basho/riak-erlang-client/pull/59

MapReduce results lists are built in reverse order to take advantage of the lower cost of `cons` as compared to `orddict:append`.