pgsql: Switch PgStat_Kind from an enum to a uint32 type

Switch PgStat_Kind from an enum to a uint32 type

A follow-up patch is planned to make cumulative statistics pluggable,
and using a type is useful in the internal routines used by pgstats as
PgStat_Kind may have a value that was not originally in the enum removed
here, once made pluggable.

While on it, this commit switches pgstat_is_kind_valid() to use
PgStat_Kind rather than an int, to be more consistent with its existing
callers.  Some loops based on the stats kind IDs are switched to use
PgStat_Kind rather than int, for consistency with the new time.

Author: Michael Paquier
Reviewed-by: Dmitry Dolgov, Bertrand Drouvot
Discussion: https://postgr.es/m/Zmqm9j5EO0I4W8dx@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3188a4582a8ce8223fba6e08636d8aaa4cbfd29f

Modified Files
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src/backend/utils/activity/pgstat.c       | 14 +++++------
src/backend/utils/activity/pgstat_shmem.c |  2 +-
src/include/pgstat.h                      | 39 +++++++++++++++----------------
3 files changed, 27 insertions(+), 28 deletions(-)