pgstat: split relation, database handling out of pgstat_report_stat().

pgstat: split relation, database handling out of pgstat_report_stat().

pgstat_report_stat() handles several types of stats, yet relation stats have
so far been handled directly in pgstat_report_stat().

A later commit will move the handling of the different kinds of stats into
separate files.  By splitting out relation handling in this commit that later
move will just move code around without other changes.

Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20220303021600.hs34ghqcw6zcokdh@alap3.anarazel.de