Upgrade Netty to 4.0.19.Final because of many performance improvements.

The are mostly done in the PooledByteBufAllocator.
Merge pull request #1630 from normanmaurer/netty_upgrade

Upgrade Netty to 4.0.19.Final because of many performance improvements.
No need to do a memory copy when using netty >= 4.0.19.

Just slice out the frame and call retain() on it, this will eliminate the need of memory copy for each frame and so improve the overall performance.