contrib/amcheck: Add heapam CHECK_FOR_INTERRUPTS().

Add a CHECK_FOR_INTERRUPTS() call to make heap relation verification
responsive to query cancellations.

Author: Mark Dilger <mark.dilger@enterprisedb.com>
Discussion: https://postgr.es/m/CAH2-Wzk-9RtQgb2QiuLv8j2O0j9tSFKPmmch5nWSZhguUxvbrw%40mail.gmail.com
Backpatch: 14-, where amcheck heap verification was introduced.

contrib/amcheck/verify_heapam.c

@@ -395,6 +395,8 @@ verify_heapam(PG_FUNCTION_ARGS)
 	{
 		OffsetNumber maxoff;
 
+		CHECK_FOR_INTERRUPTS();
+
 		/* Optionally skip over all-frozen or all-visible blocks */
 		if (skip_option != SKIP_PAGES_NONE)
 		{