chore(abstract-mapreduce): cleanup taskqueue.js (pouchdb#8891)

* chore(abstract-mapreduce): cleanup taskqueue.js

Co-authored-by: Gareth Bowen <[email protected]>

packages/node_modules/pouchdb-abstract-mapreduce/src/taskqueue.js

@@ -3,18 +3,16 @@
  * callbacks will eventually fire (once).
  */
 
-
 class TaskQueue {
   constructor() {
-    this.promise = new Promise(function (fulfill) {fulfill(); });
+    this.promise = Promise.resolve();
   }
 
   add(promiseFactory) {
-    this.promise = this.promise.catch(function () {
+    this.promise = this.promise
       // just recover
-    }).then(function () {
-      return promiseFactory();
-    });
+      .catch(() => { })
+      .then(() => promiseFactory());
     return this.promise;
   }