lnchannel: verify sig of remote chanupd (for inc edge of direct chan)

This is re the channel update for the incoming direction of our own channels.
This message can only come from the counterparty itself so maybe the sig check
is redundant... but for sanity I think we should check it anyway.

electrum/channel_db.py

@@ -569,12 +569,14 @@ class ChannelDB(SqlDB):
                 c.execute("INSERT INTO address (node_id, host, port, timestamp) VALUES (?,?,?,?)", (addr.pubkey, addr.host, addr.port, 0))
 
     @classmethod
-    def verify_channel_update(cls, payload) -> None:
+    def verify_channel_update(cls, payload, *, start_node: bytes = None) -> None:
         short_channel_id = payload['short_channel_id']
         short_channel_id = ShortChannelID(short_channel_id)
         if constants.net.rev_genesis_bytes() != payload['chain_hash']:
             raise InvalidGossipMsg('wrong chain hash')
-        if not verify_sig_for_channel_update(payload, payload['start_node']):
+        start_node = payload.get('start_node', None) or start_node
+        assert start_node is not None
+        if not verify_sig_for_channel_update(payload, start_node):
             raise InvalidGossipMsg(f'failed verifying channel update for {short_channel_id}')
 
     @classmethod