Upon calling NSURLSessionDownloadTask's resume, the task's timeout
countdown starts. When the AssetManagerEx starts downloading, it creates
all the tasks all at once and just `resume` upon creation. When the
number of asset files is very large, later tasks tend to end up in
'request timed out -1001' error before they can even start downloading.
This commit adds a buffer queue to prevent all the NSURLSessionTasks to
start all at once.