tasks: rework the startTask API

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

src/reverseproxy.js

@@ -707,7 +707,7 @@ async function startRenewCerts(options, auditSource) {
     assert.strictEqual(typeof auditSource, 'object');
 
     const taskId = await tasks.add(tasks.TASK_CHECK_CERTS, [ options, auditSource ]);
-    tasks.startTask(taskId, {});
+    safe(tasks.startTask(taskId, {}), { debug }); // background
     return taskId;
 }