Make Paging stores easier

[johnynek]

trait PagingReadableStore[K, V] extends ReadableStore[K,Spool[V]]
// see: https://github.com/twitter/util/blob/master/util-core/src/main/scala/com/twitter/concurrent/Spool.scala

//combinator:
object PagingReadableStore {
  // C is some cursor type
  def from[K,V,C](readable: ReadableStore[(K,C), (V,C)])(nextPage: C => C): PagingReadableStore[K,V] // do the get, make an item for the Spool, on .tail, do the next call
}

This would be good for a store of Uid -> Followers on Twitter. Some have millions and we can't fit them into one response.

[rubanm]

Sounds great. I think this will fit in really well with Redis list and set stores.
Can perhaps be done on MySQL as well using query LIMIT and some deduction for the cursor values.

[johnynek]

Yeah, the MySQL version would be great too. Indeed.