BUG-1132: upgrade netty to 4.0.19

As per description in bugzilla, this bumps netty version to 4.0.19, such
that we get buffer pooling performance enhancements needed for
openflowjava.

Change-Id: I0fcb537c89dd4fce2bcc28c422993cb8980c5afa
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-1132: upgrade netty to 4.0.19"