change default session_idle_timeout_secs

[HarrisChu]

refer https://discuss.nebula-graph.com.cn/t/topic/6429/17
Currently, session_idle_timeout_secs is 0 by default, It means no expiration.
If user doesn't release the session, and then it would be a lot of sessions in graphd, and may report to meta with useless sessions.

Suggest change the default value with 30min or 1hour?

[HarrisChu]

BTW:
wait_timeout in mysql is 28800 by default, it means 8 hours