Netty 4 - AioEventLoopGroup does not shutdown (termiante) underlying AIO Thread

[alex-berger]

Calling shutdown on an AioEventLoopGroup does not shutdown (terminate) the underlying AsynchronousChanngelGroup. Which is clearly a resource leak. The underlying Threads of
the AsynchronousChannelGroup will not terminate until the JVM exits.

The AsynchronousChanngelGroup created in the constructor of AioEventLoopGroup must be shutdown once it is not used anymore.

[normanmaurer]

Duplicate of #659