Generate pg_stat_get*() functions for databases using macros

Generate pg_stat_get*() functions for databases using macros

The same code pattern is repeated 21 times for int64 counters (0 for
missing entry) and 5 times for doubles (0 for missing entry) on database
entries.  This code is switched to use macros for the basic code
instead, shaving a few hundred lines of originally-duplicated code
patterns.  The function names remain the same, but some fields of
PgStat_StatDBEntry have to be renamed to cope with the new style.

This is in the same spirit as 83a1a1b.

Author: Michael Paquier
Reviewed-by: Nathan Bossart, Bertrand Drouvot
Discussion: https://postgr.es/m/Y46stlxQ2LQE20Na@paquier.xyz