Tweak writetup_heap/readtup_heap to avoid storing the tuple identity

Tweak writetup_heap/readtup_heap to avoid storing the tuple identity
and transaction visibility fields of tuples being sorted.  These are
always uninteresting in a tuple being sorted (if the fields were actually
selected, they'd have been pulled out into user columns beforehand).
This saves about 24 bytes per row being sorted, which is a useful savings
for any but the widest of sort rows.  Per recent discussion.