fix: use lifecycle instead of beforeStart

Author: fengmk2

src/egg.ts

@@ -239,9 +239,9 @@ export class EggCore extends KoaApplication {
     const singleton = new Singleton(options);
     const initPromise = singleton.init();
     if (initPromise) {
-      this.beforeStart(async () => {
+      this.lifecycle.registerBeforeStart(async () => {
         await initPromise;
-      });
+      }, `${name}-singleton-init`);
     }
   }
 

src/singleton.ts

@@ -108,7 +108,7 @@ export class Singleton<T = any> {
   createInstance(config: Record<string, any>, clientName: string) {
     // async creator only support createInstanceAsync
     assert(!isAsyncFunction(this.create),
-      `[@eggjs/core/singleton] ${this.name} only support synchronous creation, please use createInstanceAsync`);
+      `[@eggjs/core/singleton] ${this.name} only support asynchronous creation, please use createInstanceAsync`);
     // options.default will be merge in to options.clients[id]
     config = {
       ...this.options.default,