[SPARK-14547] Avoid DNS resolution for reusing connections

## What changes were proposed in this pull request?
This patch changes the connection creation logic in the network client module to avoid DNS resolution when reusing connections.

## How was this patch tested?
Testing in production. This is too difficult to test in isolation (for high fidelity unit tests, we'd need to change the DNS resolution behavior in the JVM).

Author: Reynold Xin <rxin@databricks.com>

Closes #12315 from rxin/SPARK-14547.