Make lazy_vacuum_heap_rel match lazy_scan_heap.
Make lazy_vacuum_heap_rel variable names match those from lazy_scan_heap
where that makes sense.
Extracted from a larger patch to deal with issues with how vacuumlazy.c
sets pages all-frozen.
Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-WznuNGSzF8v6OsgjaC5aYsb3cZ6HW6MLm30X0d65cmSH6A@mail.gmail.com