Use iterator_refresh backend option for some folds

 * Add iterator_refresh capability to riak_kv_elveldb_backend and
riak_kv_multi_backend
 * Handle iterator_refresh option/capability for ?FOLD_REQs in
   riak_kv_vnode
 * support iterator_refresh option for AAE (riak_kv_index_hashtree)
 * add riak_kv_vnode:fold/4
 * return handoff fold options from handoff_started
   * handoff_started has been modified to require this
 * Get value of iterator_refresh from env. Default to true.