pgsql: Split transaction data out of pgstats table counters

Split transaction data out of pgstats table counters

At backend level, PgStat_TableCounts currently stores all the per-table
pending counters in one structure. This is now split into two parts,
with the transactional part of the data moved out of PgStat_TableCounts
into its own structure labelled with an "xact" in PgStat_RelationStatus.

While on it, move truncdropped out of the counters and into
PgStat_RelationStatus so as both structs (existing PgStat_TableCounts
and new PgStat_TableCountsXact) contain only counters. The previous
state was slightly confusing: this flag is a state tracking if a
relation has been truncated at backend-level, and not a counter.

Because truncdropped no longer lives inside PgStat_TableCounts, this
commit teaches the flush callback to treat a pending truncate as
sufficient reason to flush, even when both counter structs are otherwise
all zeros.

This refactoring work is in preparation of a different patch that wants
to introduce the concept of transactional stats data flush. Compared to
all the other stats kinds, relations are an exception as they mix
transactional and non-transactional data.

Author: Sami Imseih <samimseih(dot)pg(at)gmail(dot)com>
Reviewed-by: Bertrand Drouvot <bertranddrouvot(dot)pg(at)gmail(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/aoU32VTkUtpdUewB@paquier.xyz

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/3f2f5e7c4cc5a917e77a3d5a0c4353b702aa7307

Modified Files
--------------
src/backend/utils/activity/pgstat_relation.c | 108 ++++++++++++++-------------
src/backend/utils/adt/pgstatfuncs.c | 24 +++---
src/include/pgstat.h | 42 ++++++++---
src/tools/pgindent/typedefs.list | 1 +
4 files changed, 104 insertions(+), 71 deletions(-)