Implement proper trade preimage tx fee calculations for TPU swaps

[dimxy]

The trade_preimage RPC returns estimated transaction fees for atomic swap operations.
Also, checking balance before a swap starts requires the same tx fee estimations.
Currently to estimate tx fees for TPU swap operations the legacy swap tx fee calculation functions are reused.
This may result in not quite accurate fee estimations for TPU swaps.
Instead we should implement functions which correctly calculate tx fees for TPU swap transactions.