Avoid BufferGetPage() calls in heap_update()

Avoid BufferGetPage() calls in heap_update()

BufferGetPage() isn't cheap and heap_update() calls it multiple times
when it could just save the page from a single call. Do that.
While we are at it, make separate variables for old and new page in
heap_xlog_update(). It's confusing to reuse "page" for both pages.

Author: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/CAAKRu_a%2BhO4PCptyaPR7AMZd7FjcHfOFKKJT8ouU3KedMud0tQ%40mail.gmail.com