cpp client core when meta closes connection of query config if connection threshold is hit

The problem can be reproduced on the temp branch https://github.com/XiaoMi/pegasus/tree/timeout_core

Steps:
1. build
2. start onebox (3 meta, 5 replica, connection_threshold_endpoint is set to 7 for testing)
[network]
; how many network threads for network library(used by asio)
io_service_worker_count = 4
**connection_threshold_endpoint = 7**
3. start a shell, so primary meta connection count will be 6
4. build cpp client sample, run it (query config from primary meta will hit the connection threshold)