Race condition in Solr index synchronization can delete existing documents from the index

Steps to reproduce:

Delete and create pages while the solr.indexer job runs (at startup). This is reproduced in SolrIndexerIT in which it seems this index synchronization job runs in parallel to the creation of mandatory documents.

Expected result:

Documents that have just been created aren't deleted again from the index.

Actual result:

Some documents are deleted from the index even though they still exist in the database. This is difficult to reproduce.

I suspect that this reproduces very rarely in actual XWiki installations but it could lead to missing documents in the index in particular on the first run. As we rely more and more on a complete and up-to-date Solr index this could be an issue as important refactoring operations might not work as expected.