[dbsp] Micro/macro-step support for recursive circuits

A recursive circuit operates as both an accumulator and a splitter. It accumulates changes produced by the outer circuit. Once the outer circuit's scheduler calls flush on the nested circuit, indicating that no more inputs will be received in the current macrostep, the recursive circuit starts iterating. Every iteration of the recursive circuit is also broken up into microsteps. The entire execution of the recursive circuit completes within one microstep of the outer circuit. The output is assembled into a spine (as it already is today), but instead of consolidating this spine into a single batch, we will output it chunk-by-chunk over multiple subsequent microsteps.