Improve pipeline

[AlexProgrammerDE]

This simplifies all pipeline code and ensures some listeners like the sizer are always present. The code already assumed that the sizer is always there and thus causes issues. The sizer can be deactivated still now and has pretty much no performance losses from this. The profit from this PR is that there is less logic with modifying the PR and thus developers interacting with the channel can assume specific things about the order and placements of elements in the pipeline. This will be useful once ViaVersion is supported, and it is expected that certain elements always are in the pipeline and don't change. My plan is to also always have an encryption and compression handler in the pipeline that is controlled via AttributeKeys from netty, but for that first #828 needs to be merged. So this PR only completes the goal partially, but that's fine. PR is ready for review like it is right now.