Remove expired rdataset headers from the heap

It was discovered that an expired header could sit on top of the heap
a little longer than desireable.  Remove expired headers (headers with
rdh_ttl set to 0) from the heap completely, so they don't block the next
TTL-based cleaning.

(cherry picked from commit a9383e4b95)

lib/dns/rbtdb.c

@@ -972,6 +972,10 @@ set_ttl(dns_rbtdb_t *rbtdb, rdatasetheader_t *header, dns_ttl_t newttl) {
 	} else {
 		isc_heap_decreased(heap, header->heap_index);
 	}
+
+	if (newttl == 0) {
+		isc_heap_delete(heap, header->heap_index);
+	}
 }
 
 static bool