rate-limit-ms with adaptive limiting doesn't work as expected on Ubuntu 20.04 and later

The logic for the rate limiting change used 50% queue depth as a good high water mark to engage rate limiting, but it looks like between Linux kernel 4.18 and Linux kernel 5.4 the maximum queue depth was raised to 1024 (from 128).

This results on ocserv-sm getting flooded and connections getting dropped during TLS signing.