bug: SitemapRequestList.persistState() throws when sitemap loading has finished

[barjin]

If SitemapRequestList finishes parsing the remote sitemaps and persistState() is called, it throws an exception.

This is because after parsing the sitemaps fully, the SitemapRequestList closes the internal stream with .push(null).

When persistState() is called afterwards, the contents of the stream are read into a (persisted) list. To not mutate the internal state with persistState(), we return the stream contents back to the original stream - if the stream had been closed, this causes an exception (push after EOF).