pgsql: pgstat: Split out relation stats handling from AtEO[Sub]Xact_PgS

pgstat: Split out relation stats handling from AtEO[Sub]Xact_PgStat() etc.

An upcoming patch will add additional work to these functions. To avoid the
functions getting too complicated / doing too many things at once, split out
sub-tasks into their own functions.

Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20210405092914.mmxqe7j56lsjfsej@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e1f958d759ff71a264973d13e9bc86c7db822928

Modified Files
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src/backend/postmaster/pgstat.c | 383 ++++++++++++++++++++++------------------
1 file changed, 215 insertions(+), 168 deletions(-)