Generate pg_stat_get*() functions for tables using macros

Generate pg_stat_get*() functions for tables using macros

The same code pattern is repeated 17 times for int64 counters (0 for
missing entry) and 5 times for timestamps (NULL for missing entry) on
table entries.  This code is switched to use a macro for the basic code
instead, shaving a few hundred lines of originally-duplicated code.  The
function names remain the same, but some fields of PgStat_StatTabEntry
have to be renamed to cope with the new style.

Author: Bertrand Drouvot
Reviewed-by: Nathan Bossart
Discussion: https:/postgr.es/m/20221204173207.GA2669116@nathanxps13