Turn transaction_isolation into GUC enum

Turn transaction_isolation into GUC enum

It was previously a string setting that was converted into an enum by
custom code, but using the GUC enum facility seems much simpler and
doesn't change any functionality, except that

    set transaction_isolation='default';

no longer works, but that was never documented and doesn't work with
any other transaction characteristics.  (Note that this is not the
same as RESET or SET TO DEFAULT, which still work.)

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://www.postgresql.org/message-id/457db615-e84c-4838-310e-43841eb806e5@iki.fi