Index definition change rebuilds an unlocked Lucene index on Corax when the database default is Corax #6095

Describe the bug

Since 6.21.1, ServiceControl keeps a search engine that is set on an index itself. An index without a setting of its own is left alone, also when its definition changes. RavenDB then builds the replacement with the database default. On every installation that ran 6.20.0 to 6.21.2 that default is Corax, so a definition change moves a Lucene index that has no setting of its own, for example after a reset under a Lucene default, back to Corax. On a large database, the rebuild can take days.

Steps to reproduce

1. Create an error or audit database with ServiceControl 6.20.0 to 6.21.2. The database default is Corax.
2. In RavenDB Studio, set the database default to Lucene and reset an index. The index now uses Lucene and has no search engine setting of its own.
3. Remove the Lucene database setting again, or move the database to a server where the database default is Corax.
4. Start a ServiceControl version that ships a changed definition for that index.
5. RavenDB builds ReplacementOf/<index> with Corax and swaps it in.

Relevant log output

Warn|ServiceControl.RavenDB.StartupChecks|The following RavenDB index(es) use the Corax search engine: audit/ReplacementOf/MessagesViewIndexWithFullTextSearch. ...

Additional Information

• Affected versions: 6.21.1 to 6.22.1
• Related: Migrated Lucene indexes are reset to Corax at start-up when the index is not locked #5948
• Fixed on master by Use Lucene for new and rebuilt indexes #6035

Backported to

• 6.22.2: Index definition change rebuilds an unlocked Lucene index on Corax when the database default is Corax #6091