[electrum] coinchooser bucket refactoring

Summary:
Clarify the coinchooser code by removing the keys() indirection. Store TxInput objects in the bucket (avoid calling TxInput.from_coin_dict twice.

What is currently preventing CoinChooserBase.make_tx from taking a list of TxInput coins instead of coin dicts is the lack of a height field in TxInput.
I will fix this in a separate diff.

Test Plan: send a transaction

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14560