Don't cache SLOW_MODE_WAIT in _flood_waited_requests

Closes #1600.

telethon/client/users.py

@@ -94,6 +94,8 @@ class UserMethods:
                 if utils.is_list_like(request):
                     request = request[request_index]
 
+                # SLOW_MODE_WAIT is chat-specific, not request-specific
+                if not isinstance(e, errors.SlowModeWaitError):
                     self._flood_waited_requests\
                         [request.CONSTRUCTOR_ID] = time.time() + e.seconds