rss2i.py: download: Retry single pages when OTA timeout occured

This is a reminder issue for reference. It is related to #21.

Current behaviour

The download subcommand tries to download a bunch of pages at once. When there occurred an Over-The-Air (OTA) timeout for at least one page, the download of the whole bunch of pages is retried.

Desired Behaviour

It is much faster and a lot less error-prone to retry only the missing pages.

Desired Implementation

The _query method should support iterable commands natively. This way it may retry single iterations on its own.