StoredDict: use pointers instead of path

Instead of storing its own path, each StoredDict element stores
its own key and a pointer to its parent. If a dict is removed
from the db, its parent pointer is set to None. This makes
self.path return None for all branches that have been pruned.

This passes tests/tests_json_db.py and fixes issue #10000

electrum/lnpeer.py

@@ -1263,8 +1263,7 @@ class Peer(Logger, EventListener):
             "revocation_store": {},
             "channel_type": channel_type,
         }
-        # set db to None, because we do not want to write updates until channel is saved
-        return StoredDict(chan_dict, None, [])
+        return StoredDict(chan_dict, self.lnworker.db)
 
     @non_blocking_msg_handler
     async def on_open_channel(self, payload):