adding option for custom executor

[ghost]

This allows library user to choose what executor to use for the server.
In my case, my application makes sure that there are only 2 pools default FJP and one IO thread pool (which has lots of custom telemetry and other custom logic). Most importantly it ensures that there is only one non daemon thread (main thread) everything else is daemon hence when main thread is finished all the resource deallocation tasks which were previously scheduled to Runtime.shutDownHook()... will execute and application will shut down gracefully.

However if i use this library i will end up having another main method, which prevents application from shutting down. For libraries like this i think its important to give option to choose what executors are being used etc...

This change adds option to provide custom executor to Configuration, which will be used to create netty event loop groups...

[mrniko]

Maybe it would be better to allow user set custom EventLoopGroup instead of Executor?

[ghost]

Up to you, currently i'm using the snapshot from this pull request, but i hope there will be some way to pass an executor.

[ghost]

Oh i mean some way to choose execution context. ELG seems like an implementation detail to me, not everyone know about it but everyone know what the Executor is :)