tableam: Add pg_dump support.

tableam: Add pg_dump support.

This adds pg_dump support for table AMs in a similar manner to how
tablespaces are handled. That is, instead of specifying the AM for
every CREATE TABLE etc, emit SET default_table_access_method
statements. That makes it easier to change the AM for all/most tables
in a dump, and allows restore to succeed even if some AM is not
available.

This increases the dump archive version, as a tables/matview's AM
needs to be tracked therein.

Author: Dimitri Dolgov, Andres Freund
Discussion:
    https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de
    https://postgr.es/m/20190304234700.w5tmhducs5wxgzls@alap3.anarazel.de