## What changes were proposed in this pull request?
After SPARK-27677, the shuffle client not only handles the shuffle block but also responsible for local persist RDD blocks. For better code scalability and precise semantics(as the [discussion](https://github.com/apache/spark/pull/24892#discussion_r300173331)), here we did several changes:
- Rename ShuffleClient to BlockStoreClient.
- Correspondingly rename the ExternalShuffleClient to ExternalBlockStoreClient, also change the server-side class from ExternalShuffleBlockHandler to ExternalBlockHandler.
- Move MesosExternalBlockStoreClient to Mesos package.
Note, we still keep the name of BlockTransferService, because the `Service` contains both client and server, also the name of BlockTransferService is not referencing shuffle client only.
## How was this patch tested?
Existing UT.
Closes#25327 from xuanyuanking/SPARK-28593.
Lead-authored-by: Yuanjian Li <xyliyuanjian@gmail.com>
Co-authored-by: Yuanjian Li <yuanjian.li@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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