Use caching headers while getting rss, remove unconfigurable additional delay on errors

Unfortunately, I did not break up the two changes in two different PRs, but being quite small, I hope it is not necessary.
Change 1: Honour caching headers when getting rss requests.
Change 2: Remove the hard coded additional ten minute delay for polling rss feeds again.

Change 2 hard coded additional delays on error is a PITA if you use rss to tightly couple two internal systems and want a low delay of 1 minute only.
Otoh if you use it to get rss feeds from some random news site and don't change any polling settings, you have a default polling time of 100 minutes, on error 110 minutes, this is a bit of a joke.
Otoh if you use it to get rss feeds from some random news site and don't change any polling settings, you have a default polling time of 100 minutes, on error 110 minutes, this is a bit of a joke. So making this additional thing configurable and extend the UI to edit it, add documentation to explain what it does and so on and so on I declare this obscure feature as not needed.