Fix nginx.QAT-sample.conf for high concurrency

[sjbae1999]

The asynch_mode_nginx provides high CPS for small, short-lived flows, and Intel enables QAT by default on most of Intel Xeon processors.

However, current example nginx configuration file, nginx.QAT-sample.conf is not practical real-world high-CPS workloads for the following reasons:

1. We need to execute nginx with sudo privilege (to access QAT HW):
   As a result, the nginx worker processes are limited to a soft limit of 1024 file descriptors by default.
2. High CPS workloads typically involve a large number of concurrent connections:
   In my testing, asynch_mode_nginx with the provided nginx.QAT-sample.conf supports only up to 476 concurrent connections. When the concurrency exceeds this level, nginx fails to register new event file descriptors due to file descriptor exhaustion.

Changes made

1. I added the following line to nginx.QAT-sample.conf:

worker_rlimit_nofile 1048576

This sets the file descriptor limit for nginx worker processes to 1048576, which is the default hard limit on Ubuntu 24.04.3 LTS.
With this change, I verified that asynch_mode_nginx operates correctly with up to 2K concurrent connections.