pgsql: postmaster: Rename some shutdown related PMState phase names

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: /messages/by-id/d2cd8fd3-396a-4390-8f0b-74be65e72899@iki.fi

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/28e7a9968e183a6b8dddaff5b513b0656ba349aa

Modified Files
--------------
src/backend/postmaster/postmaster.c | 48 +++++++++++++++++++------------------
src/backend/replication/README | 4 ++--
2 files changed, 27 insertions(+), 25 deletions(-)