transaction: cache address determination from output script

In order to avoid repeatedly calling get_addr_from_output_script() on
every read of the "TxOutput.address" property, determine and cache it
only whenever the output script is created/changed.

electrum/transaction.py

@@ -148,9 +148,18 @@ class TxOutput:
             return cls(scriptpubkey=bfh(addr), value=val)
         raise Exception(f"unexpected legacy address type: {_type}")
 
+    @property
+    def scriptpubkey(self) -> bytes:
+        return self._scriptpubkey
+
+    @scriptpubkey.setter
+    def scriptpubkey(self, scriptpubkey: bytes):
+        self._scriptpubkey = scriptpubkey
+        self._address = get_address_from_output_script(scriptpubkey)
+
     @property
     def address(self) -> Optional[str]:
-        return get_address_from_output_script(self.scriptpubkey)  # TODO cache this?
+        return self._address
 
     def get_ui_address_str(self) -> str:
         addr = self.address