[HORNETQ-1312] Delay shutdown of NioEventLoopGroup when using a shared NioEventLoopGroup

This minimize the overhead of destroying and creating the NioEventLoopGroup when the user heavily open/close sessions.
Merge pull request #1505 from normanmaurer/eventloop_delay_shutdown

[HORNETQ-1312] Delay shutdown of NioEventLoopGroup when using a shared N...