Too rigorous assert in reorderbuffer.c

Hi,

My colleague Alexander Lakhin has noticed an assertion failure in
reorderbuffer.c:1330. Here is a simple snippet reproducing it:

SELECT 'init' FROM pg_create_logical_replication_slot('regression_slot', 'test_decoding');

create table t(k int);
begin;
savepoint a;
alter table t alter column k type text;
rollback to savepoint a;
alter table t alter column k type bigint;
commit;

SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');

It is indeed too opinionated since cmax of a tuple is not stable; it can
be rewritten if subxact who tried to delete it later aborts (analogy
also holds for xmax). Attached patch removes it. While here, I had also
considered worthwhile to add a test involving DDL in aborted subxact as
it is interesting anyway and wasn't covered before.