There is one case where the btree is not recursively coalescing space as it should. If keys are moved from one bucket to its neighbor, the separator between them may be smaller in size than the old separator. If the utilization of the parent is brought below lowWaterMark as a result, we should call mayBalanceWithNeighbors on the parent from a safe context.

This issue doesn't impact correct functioning of the btree, but it means some buckets could be less full than we would like them to be.