Tweaks for API Nodes

Learnt one more trick to improve performanceresponse time for API nodes (do it before starting a node):
```
sudo sysctl -w net.core.somaxconn=65535
ulimit -n 65536
```
Need to document this somewhere.

Benchmark data is welcomed.

//Update:
According to https://www.nginx.com/blog/tuning-nginx/:
>* net.core.somaxconn – The maximum number of connections that can be queued for acceptance by NGINX. The default is often very low and that’s usually acceptable because NGINX accepts connections very quickly, but it can be worth increasing it if your website experiences heavy traffic. If error messages in the kernel log indicate that the value is too small, increase it until the errors stop.
>
> **Note: If you set this to a value greater than 512, change the backlog parameter to the NGINX listen directive to match.**

And there are more info about rate-limiting on the page. Also quoted here: https://github.com/bitshares/bitshares-core/issues/843#issuecomment-381352839

More info: https://steemit.com/witness-category/@someguy123/guide-optimise-your-rpc-server-for-better-performance

No target documentation file is named. Review the existing API-node documentation and the linked NGINX tuning guidance first; document the sysctl and ulimit commands, including the backlog caveat, and consider adding benchmark data if available.

Written by the indexing model from the issue text.