Summary:
Pass in `for_compaction` to `BlockBasedTableIterator` via `BlockBasedTableReader::NewIterator`.

In 7103559f, `for_compaction` was set in `BlockBasedTable::Rep` via `BlockBasedTable::SetupForCompaction`. In hindsight it was not the right decision; it also caused TSAN to complain.
Closes https://github.com/facebook/rocksdb/pull/4048

Differential Revision: D8601056

Pulled By: sagar0

fbshipit-source-id: 30127e898c15c38c1080d57710b8c5a6d64a0ab3