Download request.

Life cycle of request:

0. insert requests to ets of downloader component.
1. component'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 Spider component's ets.
   2. if failed, maybe try again.