Fixes apache#5626 (apache#5627)

If `killAfterIdleMs` is 0, time.After() function call makes memory leak.
Refer to https://golang.org/pkg/time/#After. According to the document,
the underlying Timer object is not recovered by the garbage collector if
the timer is not fired. And It seems that if the duration is 0, it will
not be released. Therefore, use one Timer object for idle time checking
explicitly instead of the time.After() function.

pulsar-function-go/pf/instance.go

@@ -69,8 +69,13 @@ func (gi *goInstance) startFunction(function function) error {
 		return err
 	}
 
+	idleDuration := getIdleTimeout(time.Millisecond * gi.context.instanceConf.killAfterIdleMs)
+	idleTimer := time.NewTimer(idleDuration)
+	defer idleTimer.Stop()
+
 CLOSE:
 	for {
+		idleTimer.Reset(idleDuration)
 		select {
 		case cm := <-channel:
 			msgInput := cm.Message
@@ -94,7 +99,7 @@ CLOSE:
 
 			gi.processResult(msgInput, output)
 
-		case <-time.After(getIdleTimeout(time.Millisecond * gi.context.instanceConf.killAfterIdleMs)):
+		case <-idleTimer.C:
 			close(channel)
 			break CLOSE
 		}
@@ -107,6 +112,7 @@ CLOSE:
 
 func (gi *goInstance) setupClient() error {
 	client, err := pulsar.NewClient(pulsar.ClientOptions{
+
 		URL: gi.context.instanceConf.pulsarServiceURL,
 	})
 	if err != nil {
@@ -271,7 +277,7 @@ func getIdleTimeout(timeoutMilliSecond time.Duration) time.Duration {
 func (gi *goInstance) setupLogHandler() error {
 	if gi.context.instanceConf.funcDetails.GetLogTopic() != "" {
 		gi.context.logAppender = NewLogAppender(
-			gi.client,                                         //pulsar client
+			gi.client, //pulsar client
 			gi.context.instanceConf.funcDetails.GetLogTopic(), //log topic
 			getDefaultSubscriptionName(gi.context.instanceConf.funcDetails.Tenant, //fqn
 				gi.context.instanceConf.funcDetails.Namespace,