Solr index synchronization on startup doesn't remove deleted objects, object properties or attachments

Steps to reproduce:

1. Create a document and add a comment on it with some unique word in it.
2. Delete the comment while preventing the Solr index to be updated. A way is to attach a debugger and put a break point org.xwiki.search.solr.internal.SolrIndexEventListener#onEvent and to prevent the execution of the listener by doing a "force return" whenever the method is called. Another way could be to unregister this event listener.
3. Restart XWiki.

Expected result:

On startup, a message "IndexerJob         - 0 documents
added, 0 deleted and 1 updated during the synchronization of the Solr index." is logged and when searching for the unique word without "Result type" filter, no documents are found.

Actual result:

While the message is logged as expected, the object can still be found in the Solr search:

This is because the synchronization on startup only compares documents and not object or object properties. While it triggers a recursive update of the documents, this update doesn't delete anything so objects and properties that don't exist anymore in the database aren't removed from the index.