WIP: Handoff-related riak-admin CLI additions

At the moment, taking cues primarily from [this document](https://docs.google.com/a/basho.com/document/d/1Qjbj6p4cppAxBkwp5yAnChVnBt8-J7HQXDKF879QaLQ/edit#heading=h.u5njhfpubbmz)