per-key actor epoch (kv679) rebased on 2.0

See original PR for discussion, rebased for easier merge (before squashing.)

This PR addresses most of the comments from the original PR at basho/riak_kv#1040
I'm curious about the choice to use `eqc_statem` when there is only one actual state? Might it be simpler to use a normal eqc property and have one of the generated inputs be a number of times to call into the `riak_kv_vnode_status` server per test iteration?