Reduce lock level for ALTER DOMAIN ... VALIDATE CONSTRAINT

Reduce lock level for ALTER DOMAIN ... VALIDATE CONSTRAINT

Reduce from ShareLock to ShareUpdateExclusivelock.  Validation during
ALTER DOMAIN ... ADD CONSTRAINT keeps using ShareLock.

Example:

    create domain d1 as int;
    create table t (a d1);
    alter domain d1 add constraint cc10 check (value > 10) not valid;

    begin;
    alter domain d1 validate constraint cc10;

    -- another session
    insert into t values (8);

Now we should still be able to perform DML operations on table t while
the domain constraint is being validated.  The equivalent works
already on table constraints.

Author: jian he <jian.universality@gmail.com>
Reviewed-by: Dilip Kumar <dilipbalaut@gmail.com>
Reviewed-by: wenhui qiu <qiuwenhuifx@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CACJufxHz92A88NLRTA2msgE2dpXpE-EoZ2QO61od76-6bfqurA%40mail.gmail.com