wallet: move get_depending_transactions to AddressSynchronizer

and resolve TODO

electrum/address_synchronizer.py

@@ -319,6 +319,14 @@ class AddressSynchronizer(PrintError):
             self.txi.pop(tx_hash, None)
             self.txo.pop(tx_hash, None)
 
+    def get_depending_transactions(self, tx_hash):
+        """Returns all (grand-)children of tx_hash in this wallet."""
+        children = set()
+        for other_hash in self.spent_outpoints[tx_hash].values():
+            children.add(other_hash)
+            children |= self.get_depending_transactions(other_hash)
+        return children
+
     def receive_tx_callback(self, tx_hash, tx, tx_height):
         self.add_unverified_tx(tx_hash, tx_height)
         self.add_transaction(tx_hash, tx, allow_unrelated=True)