[SPARK-13833] Guard against race condition when re-caching disk blocks in memory

When reading data from the DiskStore and attempting to cache it back into the memory store, we should guard against race conditions where multiple readers are attempting to re-cache the same block in memory.

This patch accomplishes this by synchronizing on the block's `BlockInfo` object while trying to re-cache a block.

(Will file JIRA as soon as ASF JIRA stops being down / laggy).

Author: Josh Rosen <joshrosen@databricks.com>

Closes #11660 from JoshRosen/concurrent-recaching-fixes.