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/lnchannel.py

@@ -773,7 +773,7 @@ class Channel(AbstractChannel):
         Logger.__init__(self)  # should be after short_channel_id is set
         self.lnworker = lnworker
         self.storage = state
-        self.db_lock = self.storage.db.lock if self.storage.db else threading.RLock()
+        self.db_lock = self.storage.lock
         self.config = {}
         self.config[LOCAL] = state["local_config"]
         self.config[REMOTE] = state["remote_config"]