pgsql: pgstat: separate "xact level" handling out of relation specific

pgstat: separate "xact level" handling out of relation specific functions.

This is in preparation of a later commit moving relation stats handling into
its own file.

Author: Andres Freund <andres@anarazel.de>
Discussion: /messages/by-id/20220303021600.hs34ghqcw6zcokdh@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d4ba8b51c76300f06cc23f4d8a41d9f7210c4866

Modified Files
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src/backend/postmaster/pgstat.c | 68 +++++++++++++++++++++--------------------
1 file changed, 35 insertions(+), 33 deletions(-)