This allows table AMs to completely suppress TOAST table creation, or
to modify the conditions under which they are created.

Patch by me.  Reviewed by Andres Freund.

Discussion: http://postgr.es/m/CA+Tgmoa4O2n=yphqD2pERUnYmUO84bH1SqMsA-nSxBGsZ7gWfA@mail.gmail.com