Returns tx_fee or None. Use server fee only if tx is unconfirmed and not mine.

Note: being fast is prioritised over completeness here. We try to avoid deserializing
      the tx, as that is expensive if we are called for the whole history. We sometimes
      incorrectly early-exit and return None, e.g. for not-all-ismine-input txs,
      where we could calculate the fee if we deserialized (but to see if we have all
      the parent txs available, we would have to deserialize first).
      More expensive but more complete alternative: wallet.get_tx_info(tx).fee
F)