AW: Various silliness in heap_getnext and related routi nes

These pointers are useless
except in the very unusual case where one steps forward and then back
in a sequential scan (for example, "FETCH 1; FETCH BACKWARD 1;" in a
cursor).

Actually I think it should be known whether a fetch backward is actually
allowed for a particular cursor, since it usually needs to be declared
as a scroll cursor explicitly.

It seems to me that this is wrongheaded. We could simplify
and speed up the normal case by maintaining only a "current" pointer,
which would be well worth the extra work in the forward/back
step case.

So it probably does pay off for a cursor, that is explicitly requested
to be scrollable, but certainly not for others, yes.

Andreas