optimise UTXO cache to avoid keeping unspendable transactions forever

[ Slight edit to use is_unspendable(), and to measure:
  As of block 391263, this cuts the number of UTXOs from 8815881 to 8313410.
  -- RR ]

iterate.c

@@ -155,12 +155,21 @@ static void add_utxo(const tal_t *tal_ctx,
 {
 	struct utxo *utxo;
 	unsigned int i;
+	unsigned int spend_count = 0;
+
+	for (i = 0; i < t->output_count; i++)
+		if (!is_unspendable(&t->output[i]))
+			spend_count++;
+
+	if (spend_count == 0)
+		return;
 
 	utxo = tal_alloc_(tal_ctx, sizeof(*utxo) + (sizeof(utxo->amount[0]) + 1)
 			  * t->output_count, false, TAL_LABEL(struct utxo, ""));
 
 	memcpy(utxo->tx, t->sha256, sizeof(utxo->tx));
-	utxo->num_outputs = utxo->unspent_outputs = t->output_count;
+	utxo->num_outputs = t->output_count;
+	utxo->unspent_outputs = spend_count;
 	utxo->height = b->height;
 	utxo->timestamp = b->b->timestamp;
 	for (i = 0; i < utxo->num_outputs; i++)