Critical Description
This is a potential durability bug:
In Log::DoAppend, we update last_entry_op_id_ before we actually write the data to disk. David thinks this might cause us to consider something durable before it is in TabletPeer. I'm not sure. Either way, we should check this, and either make it clear that this is the last assigned entry ID (not durable) or change it to not update until after fsync()ing