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
prototypes 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.wchpu5muyto5n647@alap3.anarazel.de