Make use of SW since_id field on /feed requests

[haku]

Finally make use of ianrenton/successwhale-api#25.

[haku]

done pending testing.

[haku]

<error>since_id parameter is invalid</error> on mixed feed requests, hmm...

[ianrenton]

Unfortunately this is a bit more complicated than I thought. The error comes from Twitter when the since_id provided is not in the timeline requested, so a since_id will only normally be valid for one timeline. Not sure how best to handle this.

[haku]

Hmm.... might take this out of OS for the time being then.
Edit: I have done.

[haku]

Is this worth attempting to solve, or to complex to be worth it?
Only reliable solution I can think of involves posting all the recent item IDs and last updated dates (as many as max items wanted in response) so the the server can dedup before responding.

[ianrenton]

I don't have a decent way to implement this server-side either, so I'm
happy for this one to be dropped or bumped down to lowest priority. I'm
not really sure we'd save a lot of processing time or bandwidth by
doing this anyway.