Save self user ID in session file

Should result in one less request after connecting,
as there is no longer a need to fetch the self user.

telethon/_updates/entitycache.py

@@ -9,7 +9,7 @@ class EntityCache:
         self,
         hash_map: dict = _sentinel,
         self_id: int = None,
-        self_bot: bool = False
+        self_bot: bool = None
     ):
         self.hash_map = {} if hash_map is _sentinel else hash_map
         self.self_id = self_id