commitment.go
func (c *commitment) match(server ServerID, matchIndex uint64) {
	c.Lock()
	defer c.Unlock()
	if prev, hasVote := c.matchIndexes[server]; hasVote && matchIndex > prev {
		c.matchIndexes[server] = matchIndex
		c.recalculate()
	}
}
func (c *commitment) match(server ServerID, matchIndex uint64) {
	c.Lock()
	defer c.Unlock()
	if prev, hasVote := c.matchIndexes[server]; hasVote && matchIndex > prev {
		c.matchIndexes[server] = matchIndex
		c.recalculate()
	}
}
// Match is called once a server completes writing entries to disk: either the
// leader has written the new entry or a follower has replied to an
// AppendEntries RPC. The given server's disk agrees with this server's log up
// through the given index.