Add mutex lock to nginxConfigCache in instance watcher service

Author: dhurley

internal/watcher/instance/instance_watcher_service.go

@@ -218,10 +218,10 @@ func (iw *InstanceWatcherService) checkForUpdates(
 					"error", parseErr,
 				)
 			} else {
+				iw.cacheMutex.Lock()
 				iw.sendNginxConfigContextUpdate(newCtx, nginxConfigContext)
 				proto.UpdateNginxInstanceRuntime(newInstance, nginxConfigContext)
 
-				iw.cacheMutex.Lock()
 				iw.nginxConfigCache[nginxConfigContext.InstanceID] = nginxConfigContext
 				iw.instanceCache[newInstance.GetInstanceMeta().GetInstanceId()] = newInstance
 				iw.cacheMutex.Unlock()