Ensure that seqscans check for interrupts at least once per page.

If a seqscan encounters many consecutive pages containing only dead tuples,
it can remain in the loop in heapgettup for a long time, and there was no
CHECK_FOR_INTERRUPTS anywhere in that loop.  This meant there were
real-world situations where a query would be effectively uncancelable for
long stretches.  Add a check placed to occur once per page, which should be
enough to provide reasonable response time without adding any measurable
overhead.

Report and patch by Merlin Moncure (though I tweaked it a bit).
Back-patch to all supported branches.

src/backend/access/heap/heapam.c

@@ -217,6 +217,13 @@ heapgetpage(HeapScanDesc scan, BlockNumber page)
 		scan->rs_cbuf = InvalidBuffer;
 	}
 
+	/*
+	 * Be sure to check for interrupts at least once per page.  Checks at
+	 * higher code levels won't be able to stop a seqscan that encounters
+	 * many pages' worth of consecutive dead tuples.
+	 */
+	CHECK_FOR_INTERRUPTS();
+
 	/* read page using selected strategy */
 	scan->rs_cbuf = ReadBufferExtended(scan->rs_rd, MAIN_FORKNUM, page,
 									   RBM_NORMAL, scan->rs_strategy);