fix: poller functions have to return promises

lib/plugins.js

@@ -370,7 +370,7 @@ function plugins (settings, deviceId) {
     const sql = `DELETE FROM machine_network_heartbeat h
         USING (SELECT device_id, max(created) as lastEntry FROM machine_network_heartbeat GROUP BY device_id) d
         WHERE d.device_id = h.device_id AND h.created < d.lastEntry`
-    db.none(sql)
+    return db.none(sql)
   }
 
   function isHd (tx) {