添加站内消息，修复讨论节点丢失的问题

hydro/service/cron.js

@@ -0,0 +1,33 @@
+class Cron {
+    constructor() {
+        this.tasks = [];
+    }
+
+    plan(time, task) {
+        const now = new Date();
+        if (!(time instanceof Date)) time = new Date(time);
+        this.tasks.push({
+            id: setTimeout(() => {
+                for (const i in this.tasks) {
+                    if (this.tasks[i].func === task) {
+                        this.tasks.splice(i, 1);
+                        break;
+                    }
+                }
+                task();
+            }, time - now),
+            func: task,
+        });
+    }
+
+    interval(time, task) {
+        const now = new Date();
+        if (!(time instanceof Date)) time = new Date(time);
+        this.tasks.push({
+            id: setTimeout(() => { task(); }, time - now),
+            func: task,
+        });
+    }
+}
+
+module.exports = Cron;