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).