wallet: Use newindex primitive to generate change key index

Signed-off-by: Christian Decker <decker.christian@gmail.com>

lightningd/build_utxos.c

@@ -15,7 +15,6 @@ const struct utxo **build_utxos(const tal_t *ctx,
 				u64 *change_satoshis, u32 *change_keyindex)
 {
 	u64 fee_estimate = 0;
-	u64 bip32_max_index = db_get_intvar(ld->wallet->db, "bip32_max_index", 0);
 	const struct utxo **utxos =
 	    wallet_select_coins(ctx, ld->wallet, satoshi_out, feerate_per_kw,
 				outputscriptlen,
@@ -30,8 +29,7 @@ const struct utxo **build_utxos(const tal_t *ctx,
 		*change_satoshis = 0;
 		*change_keyindex = 0;
 	} else {
-		*change_keyindex = bip32_max_index + 1;
-		db_set_intvar(ld->wallet->db, "bip32_max_index", *change_keyindex);
+		*change_keyindex = wallet_get_newindex(ld);
 	}
 	return utxos;
 }