[BUG#19384] Fix Server crash at textout

Hey hackers,

I replaced the `tupDesc_identifier` with a 64-bit signature from its
structure which was a todo. Stored Datums can detect stale definitions.

Thanks.

[1]: BUG #19384: Server crash at textout - Mailing list pgsql-bugs : Postgres Professional <https://postgrespro.com/list/id/19384-65ff412feef17f06@postgresql.org&gt;
Postgres Professional
<https://postgrespro.com/list/id/19384-65ff412feef17f06@postgresql.org&gt;