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/wallet/coin_selection.rs

@@ -569,6 +569,7 @@ mod test {
                     script_pubkey: Script::new(),
                 },
                 keychain: KeychainKind::External,
+                is_spent: false,
             }),
         }
     }
@@ -596,6 +597,7 @@ mod test {
                         script_pubkey: Script::new(),
                     },
                     keychain: KeychainKind::External,
+                    is_spent: false,
                 }),
             });
         }
@@ -615,6 +617,7 @@ mod test {
                     script_pubkey: Script::new(),
                 },
                 keychain: KeychainKind::External,
+                is_spent: false,
             }),
         };
         vec![utxo; utxos_number]

src/wallet/mod.rs

@@ -387,7 +387,13 @@ where
     /// Note that this method only operates on the internal database, which first needs to be
     /// [`Wallet::sync`] manually.
     pub fn list_unspent(&self) -> Result<Vec<LocalUtxo>, Error> {
-        self.database.borrow().iter_utxos()
+        Ok(self
+            .database
+            .borrow()
+            .iter_utxos()?
+            .into_iter()
+            .filter(|l| !l.is_spent)
+            .collect())
     }
 
     /// Returns the `UTXO` owned by this wallet corresponding to `outpoint` if it exists in the
@@ -879,6 +885,7 @@ where
                     outpoint: txin.previous_output,
                     txout,
                     keychain,
+                    is_spent: true,
                 };
 
                 Ok(WeightedUtxo {

src/wallet/tx_builder.rs

@@ -838,6 +838,7 @@ mod test {
                 },
                 txout: Default::default(),
                 keychain: KeychainKind::External,
+                is_spent: false,
             },
             LocalUtxo {
                 outpoint: OutPoint {
@@ -846,6 +847,7 @@ mod test {
                 },
                 txout: Default::default(),
                 keychain: KeychainKind::Internal,
+                is_spent: false,
             },
         ]
     }