While this commit also considerably simplifies/cleans the setup
method, the actual fix is that the networkManager is now added
to the channel pipeline with under the "packet_handler" key,
whereas before it was unnamed causing issues when transitioning
to the play state.
Allows very quick forwards and backwards jumping at the cost of only
replaying entity positions and block updates.
Still needs to be backported from 1.12.2 and contains FIXMEs
