[rss] Download only feeds that have changed.

Downloading a feed for no reason, over and over again is really bad behavior.

- Adds a new column etag to the rss_feeds table.
[rss] Add publication time to the rss_feeds table.

Adding this before the actual code that needs it, because we are migrating
to new table anyway. This can be used later to detect when an entry has been
removed or modified.
[rss] Set default poll interval to a sane value.

Polling for rss feeds every 20 seconds is overkill. If fast response times
are needed, methods other than polling should be used.