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
Nvm, I interpretted the comment about the purpose of this field incorrectly. I see now that is just is to indicate that the request for a lease has gone out, but not been fulfilled yet.