This information isn't useful to the called vnode because it already has it (its information about itself). All the information to construct the same argument passed to `handoff_starting/2` is available here but I wonder if we should just pass the worker pid. 

Unfortunately, also, I realized we don't have the vnode state which makes the callback a bit unorthodox. I think for now its fine since this is a "special callback" that most people won't need but I wondered briefly about calling into the `riak_core_vnode` and allowing it to make the callback to `handoff_started` (this last part is just a future note for myself).