chore: move mempool fee check (#5777)

Description
---
Moves the mempool fee check to before validation

Motivation and Context
---
This is a very cheap check compared to full validation. We should do
this before we check if the tx is valid so that we don't do validation
and then through away the tx because the fee is too low.

base_layer/core/src/mempool/mempool_storage.rs

@@ -77,6 +77,11 @@ impl MempoolStorage {
             .map(|k| k.excess_sig.get_signature().to_hex())
             .unwrap_or_else(|| "None?!".into());
         let timer = Instant::now();
+        // This check is almost free, so lets check this before we do any expensive validation.
+        if tx.body.get_total_fee().as_u64() < self.unconfirmed_pool.config.min_fee {
+            debug!(target: LOG_TARGET, "Tx: ({}) fee too low, rejecting",tx_id);
+            return Ok(TxStorageResponse::NotStoredFeeTooLow);
+        }
         debug!(target: LOG_TARGET, "Inserting tx into mempool: {}", tx_id);
         match self.validator.validate(&tx) {
             Ok(()) => {
@@ -88,10 +93,6 @@ impl MempoolStorage {
                 );
                 let timer = Instant::now();
                 let weight = self.get_transaction_weighting();
-                if tx.body.get_total_fee().as_u64() < self.unconfirmed_pool.config.min_fee {
-                    debug!(target: LOG_TARGET, "Tx: ({}) fee too low, rejecting",tx_id);
-                    return Ok(TxStorageResponse::NotStoredFeeTooLow);
-                }
                 self.unconfirmed_pool.insert(tx, None, &weight)?;
                 debug!(
                     target: LOG_TARGET,
@@ -104,10 +105,6 @@ impl MempoolStorage {
             Err(ValidationError::UnknownInputs(dependent_outputs)) => {
                 if self.unconfirmed_pool.contains_all_outputs(&dependent_outputs) {
                     let weight = self.get_transaction_weighting();
-                    if tx.body.get_total_fee().as_u64() < self.unconfirmed_pool.config.min_fee {
-                        debug!(target: LOG_TARGET, "Tx: ({}) fee too low, rejecting",tx_id);
-                        return Ok(TxStorageResponse::NotStoredFeeTooLow);
-                    }
                     self.unconfirmed_pool.insert(tx, Some(dependent_outputs), &weight)?;
                     Ok(TxStorageResponse::UnconfirmedPool)
                 } else {