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
Yes, I think maybe fsm_eqc_vnode is too complex even. But we need a process that starts the vnode_status_mgr and makes casts, and waits for replies, and that calls the blocking functions in vnode. So maybe just an eunit test inside the `riak_kv_vnode` module file, so it can call private functions from within a test process?