@abstractmethod<EOL><INDENT>def set_idle_timeout(self, session_key, idle_timeout):<DEDENT>
pass<EOL>
Sets the time, as a datetime.timedelta, that the associated session may
remain idle before expiring.

- A negative return value means the session will never expire.
- A non-negative return value (0 or greater) means the session
  expiration will occur if idle for that * length of time.

raises InvalidSessionException if the session has been stopped or
expired prior to calling this method