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.
I think we need a test that also checks that the live change works. Since you missed to register the setting in
`org.elasticsearch.index.settings.IndexDynamicSettingsModule` such a test should fail.