HDFS-17545. [ARR] router async rpc client.

[KeeProMise]

Description of PR

please see: https://issues.apache.org/jira/browse/HDFS-17545

How was this patch tested?

For code changes:

• [x] Does the title or this PR starts with the corresponding JIRA issue id (e.g. 'HADOOP-17799. Your PR title ...')?
• [ ] Object storage: have the integration tests been executed and the endpoint declared according to the connector-specific documentation?
• [ ] If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• [ ] If applicable, have you updated the LICENSE, LICENSE-binary, NOTICE-binary files?