func (c *commitment) setConfiguration(configuration Configuration) {
	c.Lock()
	defer c.Unlock()
	oldMatchIndexes := c.matchIndexes
	c.matchIndexes = make(map[ServerID]uint64)
	for _, server := range configuration.Servers {
		if server.Suffrage == Voter {
			c.matchIndexes[server.ID] = oldMatchIndexes[server.ID] // defaults to 0
		}
	}
	c.recalculate()
}
func (c *commitment) setConfiguration(configuration Configuration) {
	c.Lock()
	defer c.Unlock()
	oldMatchIndexes := c.matchIndexes
	c.matchIndexes = make(map[ServerID]uint64)
	for _, server := range configuration.Servers {
		if server.Suffrage == Voter {
			c.matchIndexes[server.ID] = oldMatchIndexes[server.ID] // defaults to 0
		}
	}
	c.recalculate()
}
// Called when a new cluster membership configuration is created: it will be
// used to determine commitment from now on. 'configuration' is the servers in
// the cluster.