I am only making this change as it makes it simpler to do manual testing of CPFP-ing *local* txs.
That is, by changing a few easy-to-find lines of code, I can make the GUI allow CPFP-ing a local tx, but
without this change it errors out with "unknown fee for parent transaction".
If one has an incoming local tx saved in the wallet, adb.get_tx_fee(txid) returns None
(if the tx gets into the mempool, it will use the server-reported value instead).
In contrast, wallet.get_tx_info(tx).fee calls both wallet.get_wallet_delta(tx) and adb.get_tx_fee(txid),
making it more expensive but more complete.
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