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
What are your thoughts on swapping this line and the next so that the vnode can be on about its business without waiting for a bunch of file access function calls to flow through the file server?