[SPARK-6028](https://issues.apache.org/jira/browse/SPARK-6028) uses network module to implement RPC. However, there are some configurations named with `spark.shuffle` prefix in the network module.

This PR refactors them to make sure the user can control them in shuffle and RPC separately. The user can use `spark.rpc.*` to set the configuration for netty RPC.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #9481 from zsxwing/SPARK-10745.