Adds a new config option: `WALLET_FREEZE_REUSED_ADDRESS_UTXOS`.
This is based on Bitcoin Core's "avoid_reuse" wallet flag. [0]
This opt-in feature, if enabled:
> Automatically freeze coins received to already used addresses.
> This can eliminate a serious privacy issue where a malicious user can track your spends by sending small payments
> to a previously-paid address of yours that would then be included with unrelated inputs in your future payments.
Note that currently we only have a single coinchooser policy, `CoinChooserPrivacy`,
which interacts well with this option, as it spends all coins from any selected address.
However, if we later add a different coinchooser policy, which allowed "partial spends",
care should be taken re e.g. disallowing using that when this option is set.
Also note that this PR adds this as a config option, but arguably it could be wallet-specific instead,
such as `use_change`.
[0]: https://github.com/bitcoin/bitcoin/blob/master/doc/release-notes/release-notes-0.19.0.1.md#wallet
closes https://github.com/spesmilo/electrum/issues/7497
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