Rename fields in pgstat structures for functions and relations

Rename fields in pgstat structures for functions and relations

This commit renames the members of a few pgstat structures related to
functions and relations, by respectively removing their prefix "f_" and
"t_".  The statistics for functions and relations and handled in their
own file, and pgstatfuncs.c associates each field in a structure
variable named based on the object type handled, so no information is
lost with this rename.

This will help with some of the refactoring aimed for pgstatfuncs.c, as
this makes more consistent the field names with the SQL functions
retrieving them.

Author: Bertrand Drouvot
Reviewed-by: Michael Paquier, Melanie Plageman
Discussion: https://postgr.es/m/9142f62a-a422-145c-bde0-b5bc498a4ada@gmail.com