Change default Monero mainnet node to xmr.to

It appears to be more stable.
Encountered issues with the previous setup, `monero-wallet-rpc` logs:

```
2021-05-24 04:23:54.852	E !r. THROW EXCEPTION: tools::error::no_connection_to_daemon
2021-05-24 04:23:54.857	E Exception at while refreshing, what=no connection to daemon
```

swap/src/cli/command.rs

@@ -12,7 +12,7 @@ use url::Url;
 use uuid::Uuid;
 
 // See: https://moneroworld.com/
-pub const DEFAULT_MONERO_DAEMON_ADDRESS: &str = "node.moneroworld.com:18089";
+pub const DEFAULT_MONERO_DAEMON_ADDRESS: &str = "node.xmr.to:18081";
 pub const DEFAULT_MONERO_DAEMON_ADDRESS_STAGENET: &str = "monero-stagenet.exan.tech:38081";
 
 // See: https://1209k.com/bitcoin-eye/ele.php?chain=btc