Redo Twitter Friends and Followers synclets

The old one was recursing and tying up workers, plus no one could tell what it
was doing. These new functions are way simpler and release control back to the
task queue immediately.

Though we can fetch 5000 user IDs at once, we can only fetch details on 100 at a
time, so that's our page size. At the Twitter rate limit of 350 calls/hour, that
means we'll be good on users up to 30k followers (leave some hits for tweets,
etc.) within an hour.