Disallow server start with sync_replication_slots = on and wal_level < logical.

Disallow server start with sync_replication_slots = on and wal_level < logical.

Replication slot synchronization (sync_replication_slots = on)
requires wal_level to be logical. This commit prevents the server
from starting if sync_replication_slots is enabled but wal_level
is set to minimal or replica.

Failing early during startup helps users catch invalid configurations
immediately, which is important because changing wal_level requires
a server restart.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Shveta Malik <shveta.malik@gmail.com>
Discussion: https://postgr.es/m/CAH0PTU_pc3oHi__XESF9ZigCyzai1Mo3LsOdFyQA4aUDkm01RA@mail.gmail.com