[SPARK-14699][CORE] Stop endpoints before closing the connections and don't stop client in Outbox

## What changes were proposed in this pull request?

In general, `onDisconnected` is for dealing with unexpected network disconnections. When RpcEnv.shutdown is called, the disconnections are expected so RpcEnv should not fire these events.

This PR moves `dispatcher.stop()` above closing the connections so that when stopping RpcEnv, the endpoints won't receive `onDisconnected` events.

In addition, Outbox should not close the client since it will be reused by others. This PR fixes it as well.

## How was this patch tested?

test("SPARK-14699: RpcEnv.shutdown should not fire onDisconnected events")

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #12481 from zsxwing/SPARK-14699.