Added ulimit support for RabbitMQ which was restricted to 4096 file descriptors

Likely none using this library under Debian had more than 4096 connections to RabbitMQ.

By default, this library does not control ulimit settings for RabbitMQ, fallbacking to system default which is 4096 file descriptors. This patch solves this problem under Debian and is well tested in production with over 24k connections successfully. =)
> I'm not happy with the name here, because ulimit manages much more than just file limits.
perhaps $file_limit would be a better option?

:+1: