SegmentWriter buffers child node list changes

The SegmentWriter currently buffers the list of child nodes changed on a nodestate update [0] (new node or updated node). This can be problematic in a scenario where there are a large number of children added to a node (ie. unique index size seen to spike above 10MM in one case).

To have a reference for the impact of this, at the SegmentWriter level, for a list of map entries of almost 3MM items, I saw it take up around 245MB heap.

This issue serves to track a possible improvement here in how we handle this update scenario.

[0] https://github.com/apache/jackrabbit-oak/blob/trunk/oak-segment-tar/src/main/java/org/apache/jackrabbit/oak/segment/SegmentWriter.java#L516