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 can look at some further testing on the vnode changes. 

This seems pretty tricky just due to the dependencies involved with the vnode. What do you think about using a stripped down imitation vnode (similar to `fsm_eqc_vnode`) to get some assurances about the interactions between the vnode and the status manager?