[client-app] Fix SyncbackMetadataTask dependency

Summary:
Add a sequentialId check in isDependentOnTask() so that the first
SyncbackMetadataTask won't get blocked on other SyncbackMetadataTasks
in the queue for the same pluginId.

Addresses T8112

Test Plan: Manual

Reviewers: spang, mark, juan

Reviewed By: juan

Differential Revision: https://phab.nylas.com/D4404

packages/client-app/src/flux/tasks/syncback-metadata-task.es6

@@ -18,7 +18,8 @@ export default class SyncbackMetadataTask extends SyncbackModelTask {
   isDependentOnTask(otherTask) {
     return (
       otherTask instanceof SyncbackMetadataTask &&
-      otherTask.pluginId === this.pluginId
+      otherTask.pluginId === this.pluginId &&
+      otherTask.sequentialId < this.sequentialId
     )
   }