Fix for the rate limiting problem

This avoids to fetch all the channels for the history and just obtains
the joined ones.

Closes: #440

slack.py

@@ -438,7 +438,7 @@ async def _history(self) -> None:
         log(f'Last known timestamp {dt}')
 
         chats: Sequence[IM|Channel] = []
-        chats += await self.channels() + await self.get_ims()  # type: ignore
+        chats += await self.joined_channels() + await self.get_ims()  # type: ignore
         for channel in chats:
             if isinstance(channel, Channel):
                 if not channel.is_member:
@@ -600,6 +600,36 @@ async def get_members(self, channel: str|Channel) -> set[str]:
         self._get_members_cache_cursor[id_] = r.get('response_metadata', {}).get('next_cursor')
         return self._get_members_cache[id_]
 
+    async def joined_channels(self) -> list[Channel]:
+        """
+        Returns the list of the joined slack channels.
+
+        Not cached
+        """
+        result: list[Channel] = []
+        cursor = None
+        while True:
+            r = await self.client.api_call(
+                'users.conversations',
+                cursor=cursor,
+                exclude_archived=True,
+                types='public_channel,private_channel,mpim',
+                limit=1000, # In vain hope that slack would not ignore this
+            )
+            response = self.tload(r, Response)
+
+            if response.ok:
+                conv = self.tload(r, Conversations)
+                result += conv.channels
+                # For this API, slack sends an empty string as next cursor, just to show off their programming "skillz"
+                if not conv.response_metadata or not conv.response_metadata.next_cursor:
+                    break
+                cursor = conv.response_metadata.next_cursor
+            else:
+                raise ResponseException(response.error)
+        return result
+
+
     async def channels(self, refresh: bool = False) -> list[Channel]:
         """
         Returns the list of slack channels