tasks: rework the startTask API

it is now async. change was required to reset the pending flag

src/externalldap.js

@@ -358,11 +358,7 @@ async function startSyncer() {
     if (config.provider === 'noop') throw new BoxError(BoxError.BAD_STATE, 'not enabled');
 
     const taskId = await tasks.add(tasks.TASK_SYNC_EXTERNAL_LDAP, []);
-
-    tasks.startTask(taskId, {}, function (error, result) {
-        debug('sync: done. %o %j', error, result);
-    });
-
+    safe(tasks.startTask(taskId, {}), { debug }); // background
     return taskId;
 }