Java client should use Epoll instead of Nio if available

Currently Java client is always configured to use NioEventLoopGroup. However, EpollEventLoopGroup is faster than Nio but is only available on some OS.

The client should check if Epoll is available and if it is, it should use Epoll instead of Nio.

Netty's comparison for Nio vs epoll: https://github.com/netty/netty/wiki/Native-transports