pgsql: Expose BufferUsageAccumDiff().

Expose BufferUsageAccumDiff().

Previously pg_stat_statements calculated the difference of buffer counters
by its own code even while BufferUsageAccumDiff() had the same code.
This commit expose BufferUsageAccumDiff() and makes pg_stat_statements
use it for the calculation, in order to simply the code.

This change also would be useful for the upcoming patch for the planning
counters in pg_stat_statements because the patch will add one more code
for the calculation of difference of buffer counters and that can easily be
done by using BufferUsageAccumDiff().

Author: Julien Rouhaud
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/bdfee4e0-a304-2498-8da5-3cb52c0a193e@oss.nttdata.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4a539a25ebfc48329fd656a95f3c1eb2cda38af3

Modified Files
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contrib/pg_stat_statements/pg_stat_statements.c | 26 ++-----------------------
src/backend/executor/instrument.c               |  4 +---
src/include/executor/instrument.h               |  2 ++
3 files changed, 5 insertions(+), 27 deletions(-)