pgsql: Refactor some code for ALTER TABLE SET LOGGED/UNLOGGED in tablec

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9f87da1cffda0e07ff3babcfa84abe9e849ccd95

Modified Files
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src/backend/commands/tablecmds.c | 44 +++++++++++++---------------------------
1 file changed, 14 insertions(+), 30 deletions(-)