Reducing Heap Memory Usage During Segment Construction

Segment construction is typically a heap memory intensive operation and there are opportunities to improve the same. This issue tracks the effort.

cc @Jackie-Jiang

Start by locating the segment construction entry points in Apache Pinot and trace where heap-intensive work occurs. The issue provides no file or test references, so first establish the relevant construction path and a way to verify memory usage; done means reducing heap usage during segment construction.

Written by the indexing model from the issue text.