add {refresh_iterator, true} for backend folds during handoff send
cleanup and overload handoff_started

* require options that are used in the subsequently sent ?FOLD_REQ to be
returned by the optional callback
* no longer pass SrcNode, its information the vnode can obtain elsewhere and
isn't very useful
* normalize error return values
Merge pull request #520 from basho/ajs-iterator-refresh

add {refresh_iterator, true} for backend folds during handoff send