Intermediate/Multi-versioning of Snapshots

[haraldng]

By using snapshots in the prepare phase (#38), entries or a snapshot of a server that was previously decided might be overwritten by the AcceptSync. However, we cannot determine what is decided or only accepted in this snapshot. Thus, before this new snapshot is decided, the server cannot serve decided reads.

To solve this we need to keep the old snapshot or log and serve reads with it until the new snapshot is decided.