pgsql: 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(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAAKRu_a%2BhO4PCptyaPR7AMZd7FjcHfOFKKJT8ouU3KedMud0tQ%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8d2c1df4f4c54d0a73fcabaf5e25bc55ce7fb5fa

Modified Files
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src/backend/access/heap/heapam.c | 17 ++++++++++-------
src/backend/access/heap/heapam_xlog.c | 34 ++++++++++++++++++----------------
2 files changed, 28 insertions(+), 23 deletions(-)