Remove nonmeaningful prefixes in PgStat_* fields

Hi hackers,

Please find attached a patch to $SUBJECT.

It is a preliminary patch for [1].

The main ideas are: 1) to have consistent naming between the pg_stat_get*() functions
and their associated counters and 2) to define the new macros in [1] the same way as it
has been done in 8018ffbf58 (aka not using the prefixes in the macros).

Looking forward to your feedback,

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com

[1]: https://www.postgresql.org/message-id/flat/89606d96-cd94-af74-18f3-c7ab2b684ba2@gmail.com