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
Probably need to wrap this with `ifdef(EQC)`. Perhaps add some eunit accountrements to facilitate running it with CI as well (and an `ifdef(TEST)`wrapper also if so)?