[SPARK-20994] Remove redundant characters in OpenBlocks to save memory for shuffle service.

## What changes were proposed in this pull request?

In current code, blockIds in `OpenBlocks` are stored in the iterator on shuffle service.
There are some redundant characters in  blockId(`"shuffle_" + shuffleId + "_" + mapId + "_" + reduceId`). This pr proposes to improve the footprint and alleviate the memory pressure on shuffle service.

Author: jinxing <jinxing6042@126.com>

Closes #18231 from jinxing64/SPARK-20994-v2.