wallet_db.clear_history: now clears prevouts_by_scripthash too

(which is the logical thing to do, as it too will be rebuilt as part of
the history, and the parts of it that might not be present after the
rebuild is exactly what a call to "clear_history" is supposed to get rid of)

electrum/wallet_db.py

@@ -1082,6 +1082,7 @@ class WalletDB(JsonDB):
         self.history.clear()
         self.verified_tx.clear()
         self.tx_fees.clear()
+        self._prevouts_by_scripthash.clear()
 
     def _convert_dict(self, path, key, v):
         if key == 'transactions':