Option to configure event loop group thread count

[valerauko]

While the same is configurable using a netty jvm option for client pools, for the server it's hardcoded at 2 * .availableProcessors

If there was an option to configure the event loop group thread counts (maybe even separately for the parent/child groups) users with enough understanding of their workloads would be able to configure more informed thread pools

[arnaudgeiser]

For completness, today the client can be configured by passing the io.netty.eventLoopThreads JVM property which is not the case for the server. It could be a dedicated parameter as well (or both).

There is no specific reason why this is not supported in the first place and it's rather low-hanging-fruit.
Thoughts @DerGuteMoritz ?