postmaster: Rename some shutdown related PMState phase names
The previous names weren't particularly clear. Future patches will add more
shutdown phases, making it even more important to have understandable shutdown
phases.
Suggested-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/
d2cd8fd3-396a-4390-8f0b-
74be65e72899@iki.fi