Support unbounded rivers

[EronWright]

RiverSource[T] should provide a continuous stream of data, never terminating as it does now.

Ideally we would still provide a batch mode of operation too.

1. unbounded - the paging iterator should poll the stream on the interval specified in the meta.
2. bounded - the paging iterator should read until a given point in event time, then terminate. All streams should end at the same (event) time - "now" by default.