// Copyright 2015 The Vanadium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

This is an implemetation of the Raft agreement protocol.  Each raft
member maintains a log of commands in the same order.  All commands 
go through a single master.  The master keeps track of the commit point,
i.e., the point in the log where a quorum of servers have stored the log.
Each server can apply the commands up to the commit point.  When a client
starts a member it provides a callback for applying the commands as they are
committed.  It is up to the application to make sure commands are idempotent.
For example, it can remember the last command applied and not let any old
ones be reapplied.

Raft members use the file system to persist their log records across crashes.
We're currently not syncing after writing each record to the disk to
speed things up.  Because of the idempotent callback, this will work as long
as a member of the quorum survives each master reelection but I may have to
eventualy rethink this.

The leader sends heartbeat messages at a fixed interval (hb).  Each follower will
trigger a new election if it hasn't heard from the leader in an interval 2.x * hb,
where x is a random number between 0 and 1.  The random interval reduces but does
not eliminate the likelihood of two elections starting simultaneously.

The VDL protocol is internal, i.e., it is just for raft members to talk to each
other.