[SPARK-15395][CORE] Use getHostString to create RpcAddress

## What changes were proposed in this pull request?

Right now the netty RPC uses `InetSocketAddress.getHostName` to create `RpcAddress` for network events. If we use an IP address to connect, then the RpcAddress's host will be a host name (if the reverse lookup successes) instead of the IP address. However, some places need to compare the original IP address and the RpcAddress in `onDisconnect` (e.g., CoarseGrainedExecutorBackend), and this behavior will make the check incorrect.

This PR uses `getHostString` to resolve the issue.

## How was this patch tested?

Jenkins unit tests.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #13185 from zsxwing/host-string.