Refactor some code for ALTER TABLE SET LOGGED/UNLOGGED in tablecmds.c

Refactor some code for ALTER TABLE SET LOGGED/UNLOGGED in tablecmds.c

Both sub-commands use the same routine to switch the relpersistence of a
relation, duplicated the same checks, and used a style inconsistent with
access methods and tablespaces.

SET LOGEED/UNLOGGED is refactored to avoid any duplication, setting the
reason why a relation rewrite happens within ATPrepChangePersistence().
This shaves some code.

Discussion: https://postgr.es/m/ZiiyGFTBNkqcMQi_@paquier.xyz