def unblock_username(username, pipe=None):
    """ unblock the given Username """
    do_commit = False
    if not pipe:
        pipe = REDIS_SERVER.pipeline()
        do_commit = True
    if username:
        pipe.delete(get_username_attempt_cache_key(username))
        pipe.delete(get_username_blocked_cache_key(username))
        if do_commit:
            pipe.execute()
unblock the given Username