Move remaining code from tqual.[ch] to heapam.h / heapam_visibility.c.

Move remaining code from tqual.[ch] to heapam.h / heapam_visibility.c.

Given these routines are heap specific, and that there will be more
generic visibility support in via table AM, it makes sense to move the
s to heapam.h (routines like HeapTupleSatisfiesVacuum will
not be exposed in a generic fashion, because they are too storage
specific).

Similarly, the code in tqual.c is specific to heap, so moving it into
access/heap/ makes sense.

Author: Andres Freund
Discussion: https://postgr.es/m/20180703070645[email protected]