limit the max session when authenticate

[HarrisChu]

if we have a lot of sessions, it would make meta slowly when

• update sessions
• get sessions in heartbeat. MetaClient::loadSessions()

currently, the default value for max_allowed_connections is 1<< 63 -1 per graph, it's too large.

expected:

1. use a default limit for max_allowed_sessions in meta. e.g. 1000
2. if exceed, then cannot create new session except user change the configuration or release other sessions.

refer:
https://discuss.nebula-graph.com.cn/t/topic/8042/18

[HarrisChu]

#3729