Dynamically chose fee for TxRefund and TxPunish.

Alice chooses the fee for TxPunish because she is the one that cares.
Bob chooses the fee for TxRefund because he is the one that cares.

Note must be taken here because if the fee is too low (e.g. < min tx fee) then she might not be able to publish TxRedeem at all.

swap/src/bitcoin/lock.rs

@@ -136,6 +136,7 @@ impl TxLock {
         &self,
         spend_address: &Address,
         sequence: Option<u32>,
+        spending_fee: Amount,
     ) -> Transaction {
         let previous_output = self.as_outpoint();
 
@@ -147,7 +148,8 @@ impl TxLock {
         };
 
         let tx_out = TxOut {
-            value: self.inner.clone().extract_tx().output[self.lock_output_vout()].value - TX_FEE,
+            value: self.inner.clone().extract_tx().output[self.lock_output_vout()].value
+                - spending_fee.as_sat(),
             script_pubkey: spend_address.script_pubkey(),
         };