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 <[email protected]>
Reviewed-by: Bertrand Drouvot <[email protected]>
Reviewed-by: Michael Paquier <[email protected]>
Discussion: https://postgr.es/m/[email protected]

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(-)

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