Store items.

Life cycle of request:

0. insert requests to ets of downloader.
1. downloader's producer get pass out to processes.
2. processes handle message.
   1. handle by pre pipelines.
   2. call Requester.request/3.
   3. handle by post pipelines.
3. pass out message.
   1. if success, pass out to Batchers.
   2. if failed, maybe try again.
4. batcher get enough message and call handle_batch/4.
   1. call Storage.store/3 to save items.
   2. if success, done.
   3. if failed, maybe try again.