pgsql: Rename routines for write/read of pgstats file

Rename routines for write/read of pgstats file

This commit renames write_chunk and read_chunk to respectively
pgstat_write_chunk() and pgstat_read_chunk(), along with the *_s
convenience macros.

These are made available for plug-ins, so as any code that decides to
write and/or read stats data can rely on a single code path for this
work.

Extracted from a larger patch by the same author.

Author: Sami Imseih <samimseih(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAA5RZ0s9SDOu+Z6veoJCHWk+kDeTktAtC-KY9fQ9Z6BJdDUirQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ed823da12891232a6542168a0a1b7d2d1c5f3e9a

Modified Files
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src/backend/utils/activity/pgstat.c | 56 +++++++++++++++++--------------------
src/include/utils/pgstat_internal.h | 6 ++++
2 files changed, 32 insertions(+), 30 deletions(-)