Distinguish fee from necessary_fee in monero-wallet

If there's no change, the fee is difference of the inputs to the outputs. The
prior code wouldn't check that amount is greater than or equal to the necessary
fee, and returning the would-be change amount as the fee isn't necessarily
helpful.

Now the fee is validated in such cases and the necessary fee is returned,
enabling operating off of that.

processor/src/networks/monero.rs

@@ -159,7 +159,7 @@ impl EventualityTrait for Eventuality {
 pub struct SignableTransaction(MSignableTransaction);
 impl SignableTransactionTrait for SignableTransaction {
   fn fee(&self) -> u64 {
-    self.0.fee()
+    self.0.necessary_fee()
   }
 }
 
@@ -293,7 +293,7 @@ impl Monero {
     let fee = fees.get(fees.len() / 2).copied().unwrap_or(0);
 
     // TODO: Set a sane minimum fee
-    const MINIMUM_FEE: u64 = 5_000_000;
+    const MINIMUM_FEE: u64 = 50_000;
     Ok(FeeRate::new(fee.max(MINIMUM_FEE), 10000).unwrap())
   }
 
@@ -383,7 +383,7 @@ impl Monero {
     ) {
       Ok(signable) => Ok(Some({
         if calculating_fee {
-          MakeSignableTransactionResult::Fee(signable.fee())
+          MakeSignableTransactionResult::Fee(signable.necessary_fee())
         } else {
           MakeSignableTransactionResult::SignableTransaction(signable)
         }
@@ -405,17 +405,17 @@ impl Monero {
         SendError::MultiplePaymentIds => {
           panic!("multiple payment IDs despite not supporting integrated addresses");
         }
-        SendError::NotEnoughFunds { inputs, outputs, fee } => {
+        SendError::NotEnoughFunds { inputs, outputs, necessary_fee } => {
           log::debug!(
-            "Monero NotEnoughFunds. inputs: {:?}, outputs: {:?}, fee: {fee:?}",
+            "Monero NotEnoughFunds. inputs: {:?}, outputs: {:?}, necessary_fee: {necessary_fee:?}",
             inputs,
             outputs
           );
-          match fee {
-            Some(fee) => {
+          match necessary_fee {
+            Some(necessary_fee) => {
               // If we're solely calculating the fee, return the fee this TX will cost
               if calculating_fee {
-                Ok(Some(MakeSignableTransactionResult::Fee(fee)))
+                Ok(Some(MakeSignableTransactionResult::Fee(necessary_fee)))
               } else {
                 // If we're actually trying to make the TX, return None
                 Ok(None)