Never delete spent utxos from the database

A `is_spent` field is added to LocalUtxo; when a txo is spent we set
this field to true instead of deleting the entire utxo from the
database.
This allows us to create txs double-spending txs already in blockchain.
Listunspent won't return spent utxos, effectively excluding them from the
coin selection and balance calculation

src/types.rs

@@ -131,6 +131,8 @@ pub struct LocalUtxo {
     pub txout: TxOut,
     /// Type of keychain
     pub keychain: KeychainKind,
+    /// Whether this UTXO is spent or not
+    pub is_spent: bool,
 }
 
 /// A [`Utxo`] with its `satisfaction_weight`.