First steps towards index scans with heap access decoupled from index

First steps towards index scans with heap access decoupled from index
access: define new index access method functions 'amgetmulti' that can
fetch multiple TIDs per call.  (The functions exist but are totally
untested as yet.)  Since I was modifying pg_am anyway, remove the
no-longer-needed 'rel' parameter from amcostestimate functions, and
also remove the vestigial amowner column that was creating useless
work for Alvaro's shared-object-dependencies project.
Initdb forced due to changes in pg_am.