Introduces expert only event loop thread count setting.

[mashhurs]

Description

By default, netty event loop creates available_processors * 2 thread count. This may sometimes lead a resource problem, especially when multiple beats-input pipelines are defined in a single machine.
This PR introduces a configuration to limit a number of threads for netty event loop threads.

How to locally test

• unfortunately I couldn't find a better way to add a unit test for this feature
• pull the change and run pipeline with following config
• connect, reconnect multiple times the client to the downstream beats-input.
• check the threads (with thread dump tools or ls diagnostics tool) that two (or defined) threads will created for netty event loop named as nioEventLoopGroup-M-N

input {
    beats {
        codec => json
        port => 5001
        event_loop_threads => 2
        executor_threads => 2
    }
}
output {
    stdout { codec => rubydebug }
}

[robbavey]

Code LGTM, some doc changes requested

[robbavey]

LGTM