rss input uses ttl instead of intervall

[arosenheinrich]

RSS channel feature an optional ttl element that indicates how long a channel can be cached before it should be refreshed from the source. It would be an interesting option to use this instead of the intervall parameter for the rss input. My idea would be to make intervall an optional parameter, in case it is not set the channel will be fetched once and, if available, ttl will be used as recrawl time, otherwise a default value could be used.

This is actually the behavior of the elasticsearch rss river (http://dadoonet.github.com/rssriver/), it would be cool to have the same feature in the rss input.

Thanks in advance!
Andrej