[dataflow] Arrangement -> Stream operator is not fueled

[frankmcsherry]

The operator that turns an arrangement into a stream of updates does not use "fuel", in the sense that it will run to completion even if that involves a substantial amount of data production. This operator is a bit like a source, in that it produces dataflow data "from nothing", and not fueling things can result in substantial bursts of work for other operators. Adding fuel based limiting has the potential to reduce situations in which MZ overwhelms itself.