fix(watchdog): Close dangling file descriptors on shutdown and config reload (#544)

* fix(watchdog): Add functions to avoid dangling file descriptors

* Change function name and add comment under core/endpoint.go
- change the function name of CloseHTTPConnection() to Close()
- add some comments above Close() function

* Update core/endpoint.go

* Update core/endpoint.go

---------

Co-authored-by: Richard Cheng <richard_cheng@trendmicro.com>
Co-authored-by: TwiN <twin@linux.com>

watchdog/watchdog.go

@@ -96,6 +96,10 @@ func UpdateEndpointStatuses(endpoint *core.Endpoint, result *core.Result) {
 }
 
 // Shutdown stops monitoring all endpoints
-func Shutdown() {
+func Shutdown(cfg *config.Config) {
+	// Disable all the old HTTP connections
+	for _, endpoint := range cfg.Endpoints {
+		endpoint.Close()
+	}
 	cancelFunc()
 }