Add a new 'F' entry type for fixed-numbered stats in pgstats file

Add a new 'F' entry type for fixed-numbered stats in pgstats file

This new entry type is used for all the fixed-numbered statistics,
making possible support for custom pluggable stats.  In short, we need
to be able to detect more easily if a stats kind exists or not when
reading back its data from the pgstats file without a dependency on the
order of the entries read.  The kind ID of the stats is added to the
data written.

The data is written in the same fashion as previously, with the
fixed-numbered stats first and the dshash entries next.  The read part
becomes more flexible, loading fixed-numbered stats into shared memory
based on the new entry type found.

Bump PGSTAT_FILE_FORMAT_ID.

Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/Zot5bxoPYdS7yaoy@paquier.xyz