pgstat: move transactional code into pgstat_xact.c.
The transactional integration code is largely independent from the rest of
pgstat.c. Subsequent commits will add more related code.
Author: Andres Freund <andres@anarazel.de>
Reviewed-By: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/
20220404041516.cctrvpadhuriawlq@alap3.anarazel.de