postmaster: Update pmState via a wrapper function

postmaster: Update pmState via a wrapper function

This makes logging of state changes easier - state transitions are now logged
at DEBUG1. Without that logging it was surprisingly hard to understand the
current state of the system while debugging.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/kgng5nrvnlv335evmsuvpnh354rw7qyazl73kdysev2cr2v5zu@m3cfzxicm5kp