Celery worker, Limit frames tracked with Xmem

[MattGrouchy]

Related to pacefactory/expresso_server#79

[MattGrouchy]

  track_frame_batch_limit: int = 10
    """ Max number of frames to track in a batch with Xmem. Xmem uses RAM linearly with the number of frames that are
     tracked. Limiting the number of frames to track in one batch, restricts the total amount of RAM used. """