Aggregate and emit until custom boundary condition met.
This operator can be customized into a broad class of aggregate/group/fold operators, based on custom state or timer conditions.
Reactive Streams semantics
emits when the aggregation function decides the aggregate is complete or the timer function returns true
backpressures when downstream backpressures and the aggregate is complete
completes when upstream completes and the last aggregate has been emitted downstream
cancels when downstream cancels