pgsql: Add pg_memory_is_all_zeros() in memutils.h

Add pg_memory_is_all_zeros() in memutils.h

This new function tests if a memory region starting at a given location
for a defined length is made only of zeroes.  This unifies in a single
path the all-zero checks that were happening in a couple of places of
the backend code:
- For pgstats entries of relation, checkpointer and bgwriter, where
some "all_zeroes" variables were previously used with memcpy().
- For all-zero buffer pages in PageIsVerifiedExtended().

This new function uses the same forward scan as the check for all-zero
buffer pages, applying it to the three pgstats paths mentioned above.

Author: Bertrand Drouvot
Reviewed-by: Peter Eisentraut, Heikki Linnakangas, Peter Smith
Discussion: https://postgr.es/m/ZupUDDyf1hHI4ibn@ip-10-97-1-34.eu-west-3.compute.internal

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/07e9e28b56db4d8533b62b3a921f1e74aae26b1f

Modified Files
--------------
src/backend/storage/page/bufpage.c               | 13 +------------
src/backend/utils/activity/pgstat_bgwriter.c     |  5 +++--
src/backend/utils/activity/pgstat_checkpointer.c |  7 +++----
src/backend/utils/activity/pgstat_relation.c     |  7 ++-----
src/include/utils/memutils.h                     | 17 +++++++++++++++++
5 files changed, 26 insertions(+), 23 deletions(-)