addr_sync: update "stored_height" db field immediately on wallet-open

Repro steps:
- in qt gui, with network enabled, open wallet1
- open wizard, create wallet2 (restore from seed something that has mined history)
- close both wallets, stop electrum
- start electrum with "-o" offline flag, open wallet2
- observe all txs in history tab show up as "unconfirmed"

The cause is that "stored_height" only gets updated ~on new blocks.
So if you created a wallet and closed it soon, its db would not contain "stored_height."

electrum/address_synchronizer.py

@@ -208,12 +208,13 @@ class AddressSynchronizer(Logger, EventListener):
             self.verifier = SPV(self.network, self)
             self.asyncio_loop = network.asyncio_loop
             self.register_callbacks()
+            self._update_stored_local_height()
 
     @event_listener
     @with_lock
     def on_event_blockchain_updated(self, *args):
         self.invalidate_cache()
-        self.db.put('stored_height', self.get_local_height())
+        self._update_stored_local_height()
 
     async def stop(self):
         if self.network:
@@ -694,6 +695,9 @@ class AddressSynchronizer(Logger, EventListener):
             return cached_local_height
         return self.network.get_local_height() if self.network else self.db.get('stored_height', 0)
 
+    def _update_stored_local_height(self) -> None:
+        self.db.put('stored_height', self.get_local_height())
+
     def set_future_tx(self, txid: str, *, wanted_height: int):
         """Mark a local tx as "future" (encumbered by a timelock).
         wanted_height is the min (abs) block height at which the tx can get into the mempool (be broadcast).