Add a method to cancel_all conversations (#1183)

telethon/client/telegrambaseclient.py

@@ -1,5 +1,6 @@
 import abc
 import asyncio
+import collections
 import logging
 import platform
 import time
@@ -322,8 +323,9 @@ class TelegramBaseClient(abc.ABC):
 
         # Some further state for subclasses
         self._event_builders = []
-        self._conversations = {}
-        self._ids_in_conversations = {}  # chat_id: count
+
+        # {chat_id: {Conversation}}
+        self._conversations = collections.defaultdict(set)
 
         # Default parse mode
         self._parse_mode = markdown