Core: added a warning about reusing connections. Previously, reusing connections happened silently and was only visible in monitoring systems. This was shown to be not very user-friendly, and administrators often didn't realize there were too few connections available to withstand the load, and configured timeouts (keepalive_timeout and http2_idle_timeout) were effectively reduced to keep things running. To provide at least some information about this, a warning is now logged (at most once per second, to avoid flooding the logs).

--- a/src/core/ngx_cycle.h
+++ b/src/core/ngx_cycle.h
@@ -55,6 +55,7 @@ struct ngx_cycle_s {
 
     ngx_queue_t               reusable_connections_queue;
     ngx_uint_t                reusable_connections_n;
+    time_t                    connections_reuse_time;
 
     ngx_array_t               listening;
     ngx_array_t               paths;