Add per key actor epochs

See riak_kv#679 and the associated platform_task RFC and summary.

Requires https://github.com/basho/riak_core/pull/646

This PR addresses the "doomstone", backup-restore, and some byzantine flavours of the kv679 bug.

The RFC explains the mechanism in detail but briefly:

1. add a persisted to disk vnode counter (persisted with leases, aysnc)
2. when ever a key is written to for the "first time" by a vnode, create an epoch actor for the key by concatenating vnodeid+counter (and increment the counter)

This ensures that a first time write for a key gets a new actor, this is the epoch for the key. It means we don't mix up deleted+re-created keys {a,1} event with the original {a, 1} event for some key, by ensuring an actor per epoch, without causing a keyspace wide actor explosion.

I hope that is enough details for the review to start. I'll start work on EQC for the status_mgr and the new vnode functions in the mean time.

The empty list is not strictly necessary in this type. Also, is there any case where you will have bare atoms in the list? If not, `orddict:orddict()` might be better all around.