(WIP) Migration to Netty 4

Hi,

I wanted to migrate from old Netty 3.x to 4.1.x, which might offer some performance and memory enhancements, see https://netty.io/wiki/new-and-noteworthy-in-4.0.html and https://blog.twitter.com/engineering/en_us/a/2013/netty-4-at-twitter-reduced-gc-overhead.html.

It appears that the migration is tougher than I thought, and I cannot make it work : Play is stuck when starting the application.
Nevertheless, I think I still have done a big part of the work, replacing deprecated calls and updated new API calls. I removed all compilation errors and there is no error on startup. I guess I lack the necessary understanding to solve the remaining issues, so I just want to share the work in progress and welcome any help to finish the job !

The changes so far have been commited in this branch : https://github.com/tomparle/play1/tree/libupgrade . Tell me if it should be transformed in pull request or anything else.

Review the libupgrade branch and the migration changes from Netty 3.x to 4.1.x, starting with where Play becomes stuck during application startup. Done means the migration completes without compilation errors and Play starts the application successfully on Netty 4.1.x.