Replace hardcoded identifiers of pgstats file by #defines

Replace hardcoded identifiers of pgstats file by #defines

This changes pgstat.c so as the three types of entries that can exist in
a pgstats file are not hardcoded anymore, replacing them with
descriptively-named macros, when reading and writing stats files:
- 'N' for named entries, like replication slot stats.
- 'S' for entries identified by a hash.
- 'E' for the end-of-file

This has come up while working on making this area of the code more
pluggable.  The format of the stats file is unchanged, hence there is no
need to bump PGSTAT_FILE_FORMAT_ID.

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