Re: HOT chain validation in verify_heapam()

Currently the new verify_heapam() follows ctid chains when XMAX_INVALID is set
and expects to find an item it can dereference - but I don't think that's
something we can rely on: Afaics HOT pruning can break chains, but doesn't
reset xmax.

                        /*
                         * If the next line pointer is a redirect, or if it's a tuple
                         * but the XMAX of this tuple doesn't match the XMIN of the next
                         * tuple, then the two aren't part of the same update chain and
                         * there is nothing more to do.
                         */
                        if (ItemIdIsRedirected(next_lp))
                                continue;
                        curr_htup = (HeapTupleHeader) PageGetItem(ctx.page, curr_lp);
                        curr_xmax = HeapTupleHeaderGetUpdateXid(curr_htup);
                        next_htup = (HeapTupleHeader) PageGetItem(ctx.page, next_lp);
                        next_xmin = HeapTupleHeaderGetXmin(next_htup);
                        if (!TransactionIdIsValid(curr_xmax) ||
                                !TransactionIdEquals(curr_xmax, next_xmin))
                                continue;