go-micro selector 选择器

　　selecor是一种基于registry的选择服务节点和标记它们状态的的机制，有Random、RoundRobin两种策略，代码如下：

// Selector builds on the registry as a mechanism to pick nodes
// and mark their status. This allows host pools and other things
// to be built using various algorithms.
type Selector interface {
	Init(opts ...Option) error
	Options() Options
	// Select returns a function which should return the next node
	Select(service string, opts ...SelectOption) (Next, error)
	// Mark sets the success/error against a node
	Mark(service string, node *registry.Node, err error)
	// Reset returns state back to zero for a service
	Reset(service string)
	// Close renders the selector unusable
	Close() error
	// Name of the selector
	String() string
}

// Next is a function that returns the next node
// based on the selector's strategy
type Next func() (*registry.Node, error)

// Filter is used to filter a service during the selection process
type Filter func([]*registry.Service) []*registry.Service

// Strategy is a selection strategy e.g random, round robin
type Strategy func([]*registry.Service) Next

　　

　　cacheSelector：

　　1、以本地缓存作为服务节点信息的存储，可以快速提供节点信息

　　2、字段 ttl 作为缓存时间，默认是1分钟；过期了则重新去registry获取