[7.x] Make LazyCollection#countBy be lazy#33801
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The collection's current
countByimplementation usesgroupBy.groupByis inherently not lazy (being a form of bucket sort). This means that in order to count occurrences, we need to pull the whole dataset into memory at once.We can do better.
The code in this PR only pulls a single item at a time, and increases the counter. The actual item is then promptly discarded, so that we never keep more than a single item in memory.