Core: idle connections now closed only once on exiting. Iterating through all connections takes a lot of CPU time, especially with large number of worker connections configured. As a result nginx processes used to consume CPU time during graceful shutdown. To mitigate this we now only do a full scan for idle connections when shutdown signal is received. Transitions of connections to idle ones are now expected to be avoided if the ngx_exiting flag is set. The upstream keepalive module was modified to follow this.