Use defer style to stop tickers to avoid current/possible misuse (#5883)

This commit ensures that all tickers are stopped using defer ticker.Stop()
style. This will also fix one bug seen when a client starts to listen to
event notifications and that case will result a leak in tickers.

pkg/event/target/httpclient.go

@@ -62,8 +62,10 @@ func (target *HTTPClientTarget) start() {
 			return nil
 		}
 
+		keepAliveTicker := time.NewTicker(500 * time.Millisecond)
+		defer keepAliveTicker.Stop()
+
 		for {
-			keepAliveTicker := time.NewTicker(500 * time.Millisecond)
 			select {
 			case <-target.stopCh:
 				// We are asked to stop.