Add index.checksum_on_merge option

This just exposes the current lucene option `checkIntegrityAtMerge` from LiveIndexWriterConfig. When enabled, all parts of the index are verified before merging.
Yes, I really wanted this too. I was beaten back by the fact that tests for any of the other IndexWriter options are integration tests (indexing stuff and looking at number of segments etc), instead of simply being unit tests that check that IWC was changed.