[SPARK-18188] add checksum for blocks of broadcast

## What changes were proposed in this pull request?

A TorrentBroadcast is serialized and compressed first, then splitted as fixed size blocks, if any block is corrupt when fetching from remote, the decompression/deserialization will fail without knowing which block is corrupt. Also, the corrupt block is kept in block manager and reported to driver, so other tasks (in same executor or from different executor) will also fail because of it.

This PR add checksum for each block, and check it after fetching a block from remote executor, because it's very likely that the corruption happen in network. When the corruption happen, it will throw the block away and throw an exception to fail the task, which will be retried.

Added a config for it: `spark.broadcast.checksum`, which is true by default.

## How was this patch tested?

Existing tests.

Author: Davies Liu <davies@databricks.com>

Closes #15935 from davies/broadcast_checksum.

(cherry picked from commit 7d5cb3af)
Signed-off-by: Shixiong Zhu <shixiong@databricks.com>