pkg/client/event event包

一. pkg/client/event 通过event包可以访问Fabric网络上的通道事件。 事件客户端接收诸如块，过滤块，链代码和事务状态事件之类的事件。

1. 基本流程

   1）准备频道客户端上下文
   2）创建事件客户端
   3）注册事件
   4）处理事件（或超时）
   5）取消注册

    1 ec, err := New(mockChannelProvider("mychannel"))
    2 if err != nil {
    3     fmt.Println("failed to create client")
    4 }
    5 
    6 registration, notifier, err := ec.RegisterChaincodeEvent("examplecc", "event123")
    7 if err != nil {
    8     fmt.Println("failed to register chaincode event")
    9 }
   10 defer ec.Unregister(registration)
   11 
   12 select {
   13 case ccEvent := <-notifier:
   14     fmt.Printf("received chaincode event %v\n", ccEvent)
   15 case <-time.After(time.Second * 5):
   16     fmt.Println("timeout while waiting for chaincode event")
   17 }
   18 
   19 // Timeout is expected since there is no event producer

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