This moves the responsibility for:
- creating the storage necessary for a relation, including creating a
  new relfilenode for a relation with existing storage
- non-transactional truncation of a relation
- VACUUM FULL / CLUSTER's rewrite of a table
below tableam.

This is fairly straight forward, with a bit of complexity smattered in
to move the computation of xid / multixid horizons below the AM, as
they don't make sense for every table AM.

Author: Andres Freund
Discussion: https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de