Shutdown Executor on MultithreadEventLoopGroup shutdown. Fixes #2837

Motivation:

Currently the Executor created by (Nio|Epoll)EventLoopGroup is not correctly shutdown.
This might lead to resource shortages, due to resources not being freed asap.

Modifications:

If (Nio|Epoll)EventLoopGroup create their internal Executor via a constructor
provided `ExecutorServiceFactory` object or via
MultithreadEventLoopGroup.newDefaultExecutorService(...) the ExecutorService.shutdown()
method will be called after (Nio|Epoll)EventLoopGroup is shutdown.

ExecutorService.shutdown() will not be called if the Executor object was passed
to the (Nio|Epoll)EventLoopGroup (that is, it was instantiated outside of Netty).

Result:

Correctly release resources on (Nio|Epoll)EventLoopGroup shutdown.