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