wallet: save num_parents of utxos in wallet file.

fixes #8361

electrum/wallet.py

@@ -339,6 +339,7 @@ class Abstract_Wallet(ABC, Logger, EventListener):
         self._receive_requests      = db.get_dict('payment_requests')  # type: Dict[str, Request]
         self._invoices              = db.get_dict('invoices')  # type: Dict[str, Invoice]
         self._reserved_addresses   = set(db.get('reserved_addresses', []))
+        self._num_parents          = db.get_dict('num_parents')
 
         self._freeze_lock = threading.RLock()  # for mutating/iterating frozen_{addresses,coins}
 
@@ -472,6 +473,7 @@ class Abstract_Wallet(ABC, Logger, EventListener):
     def clear_tx_parents_cache(self):
         with self.lock, self.transaction_lock:
             self._tx_parents_cache.clear()
+            self._num_parents.clear()
             self._last_full_history = None
 
     @event_listener
@@ -887,7 +889,9 @@ class Abstract_Wallet(ABC, Logger, EventListener):
     def get_num_parents(self, txid: str) -> Optional[int]:
         if not self.is_up_to_date():
             return
-        return len(self.get_tx_parents(txid))
+        if txid not in self._num_parents:
+            self._num_parents[txid] = len(self.get_tx_parents(txid))
+        return self._num_parents[txid]
 
     def get_tx_parents(self, txid: str) -> Dict[str, Tuple[List[str], List[str]]]:
         """