Fix ALTER DOMAIN VALIDATE CONSTRAINT locking

Fix ALTER DOMAIN VALIDATE CONSTRAINT locking

Commit 16a0039dc0d1 reduced the lock level for ALTER DOMAIN ...
VALIDATE CONSTRAINT from ShareLock to ShareUpdateExclusiveLock.
However, that change was unsafe. If DML on tables using the domain had
already started and initialized domain constraint checks before a NOT
VALID constraint was added, it could still insert or update rows that
violated the new constraint.

This commit reverts commit 16a0039dc0d1 so that ALTER DOMAIN ...
VALIDATE CONSTRAINT once again acquires ShareLock on relations using
the domain. Also add an isolation test covering this case.

Author: Chao Li <lic@highgo.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://postgr.es/m/463C0E1A-4A40-4BCA-839C-9236B80D65EE@gmail.com