[SPARK-18375][SPARK-18383][BUILD][CORE] Upgrade netty to 4.0.42.Final

## What changes were proposed in this pull request?

One of the important changes for 4.0.42.Final is "Support any FileRegion implementation when using epoll transport netty/netty#5825".
In 4.0.42.Final, `MessageWithHeader` can work properly when `spark.[shuffle|rpc].io.mode` is set to epoll

## How was this patch tested?

Existing tests

Author: Guoqiang Li <witgo@qq.com>

Closes #15830 from witgo/SPARK-18375_netty-4.0.42.

(cherry picked from commit bc41d997)
Signed-off-by: Sean Owen <sowen@cloudera.com>