Add overload info to vnode status response

riak_kv_console expects a list of status responses for a vnode_status
request. This change has the overload return fit the structure
of non-overload returns. This way a single overloaded vnode
doesn't stop all status messages.

Also noticed that a preflist entry of {partion(), pid()} does not
get dropped by overload handling, while {partition(), node()} does.

Changes riak_kv_status to use the overload droppable style, should
I have?