Add maxNumber and maxTime to AccumulateWhileUnchanged

[LysanderGG]

AccumulateWhileUnchanged only emits when the property changes, which can be an issue if the property doesn't change very often. It would be similar to groupedWithin, becoming Chunk up this stream into groups of elements received within a time window, or limited by the number of the elements, or by the property changing, whatever happens first.

I already customized AccumulateWhileUnchanged for a project, so I can open a PR if someone else thinks it can be useful.