message: wallet: Move discard feerate fetching to CreateTransaction
  Instead of fetching the discard feerate for each SelectCoinsMinConf
  iteration, fetch and cache it once during CreateTransaction so that it
  is shared for each SelectCoinsMinConf through
  coin_selection_params.m_discard_feerate.
  Does not change behavior.
diff: @@ -598,6 +598,7 @@ struct CoinSelectionParams
  size_t change_output_size = 0;
  size_t change_spend_size = 0;
  CFeeRate m_long_term_feerate;
  + CFeeRate m_discard_feerate;
  size_t tx_noinputs_size = 0;
  //! Indicate that we are subtracting the fee from outputs
  bool m_subtract_fee_outputs = false;
  @@ -608,6 +609,7 @@ struct CoinSelectionParams
  use_bnb(use_bnb),
  change_output_size(change_output_size),
  change_spend_size(change_spend_size),
  + m_discard_feerate(discard_feerate),
  tx_noinputs_size(tx_noinputs_size),
  m_avoid_partial_spends(avoid_partial)
  {}
  