Refactoring operation should wait for the Solr index to be empty before proceeding

Before XWIKI-19352, link indexing was done synchronously during the document save and therefore this issue didn't exist. Thus, this is actually a regression. When link indexing was made asynchronous (even before the move to Solr), it was discussed on the forum that document move operations should wait for the indexing to be complete. However, apparently, this was never implemented.