wallet: add new config option "FREEZE_REUSED_ADDRESS_UTXOS"

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

electrum/address_synchronizer.py

@@ -966,8 +966,14 @@ class AddressSynchronizer(Logger, EventListener):
         return coins
 
     def is_used(self, address: str) -> bool:
+        """Whether any tx ever touched `address`."""
         return self.get_address_history_len(address) != 0
 
+    def is_used_as_from_address(self, address: str) -> bool:
+        """Whether any tx ever spent from `address`."""
+        received, sent = self.get_addr_io(address)
+        return len(sent) > 0
+
     def is_empty(self, address: str) -> bool:
         coins = self.get_addr_utxo(address)
         return not bool(coins)