Allow backlog parameter to be set with ssl_bind DSL

[dalibor]

Description

We're adding option for setting the socket backlog value with the ssl_bind DSL, and updating the docs with a hint to the net.core.somaxconn sysctl value. What's interesting here is that on older Linux kernels it defaults to 128 (reference), so the actual backlog value is capped to it.

somaxconn - INTEGER
Limit of socket listen() backlog, known in userspace as SOMAXCONN. Defaults to 4096. (Was 128 before linux-5.4) See also tcp_max_syn_backlog for additional tuning for TCP sockets.

Even with the default backlog value of 1024 (when kernel allows it), we are seeing some requests get dropped (504 errors with AWS ALB). That happens with a super fast end-points that respond in < 5ms occasionally when receiving a blast of requests within few seconds and there are other slower end-points that saturate the queue causing some requests to get dropped. So, I'm tweaking slightly the original doc about backlog that were misleading:

Generally, you'll never hit the backlog cap in production.

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[nateberkopec]

Makes total sense. Not having this set on ssl_bind was an oversight and I agree with your docs changes as well.