sftp: only rebuild when app task queue is empty

when multiple apptasks are scheduled, we end up with a sequence like this:
    - task1 finishes
    - task2 (uninstall) removes  appdata directory
    - sftp rebuild (from task1 finish)
    - task2 fails because sftp rebuild created empty appdata directory

a fix is to delay the sftp rebuild until all tasks are done. of course,
the same race is still there, if a user initiated another task immediately
but this seems unlikely. if that happens often, we can further add a sftpRebuildInProgress
flag inside apptaskmanager.

src/apptaskmanager.js

@@ -79,14 +79,16 @@ function scheduleTask(appId, taskId, options, callback) {
         delete gActiveTasks[appId];
         locker.unlock(locker.OP_APPTASK); // unlock event will trigger next task
 
-        // post app task hooks
-        services.rebuildService('sftp', error => { if (error) debug('Unable to rebuild sftp:', error); });
         scheduler.resumeJobs(appId);
     });
 }
 
 function startNextTask() {
-    if (gPendingTasks.length === 0) return;
+    if (gPendingTasks.length === 0) {
+        // rebuild sftp when task queue is empty. this minimizes risk of sftp rebuild overlapping with other app tasks
+        services.rebuildService('sftp', error => { if (error) debug('Unable to rebuild sftp:', error); });
+        return;
+    }
 
     assert(Object.keys(gActiveTasks).length < TASK_CONCURRENCY);