SolrIndex - improve flush method

[rsoika]

currently the method flushEventLogByCount reads the eventLog entries sequencly and update or delete each document in a single Rest API call.
We can improve the performance if we collect all updates and deletions and call the solr rest api only twice - one for update and one for deletion.

Therefor we need to collect a distinct list for deletion and updates.

This is currently implemented in the imixs-microprofile servcie 'IndexService' - we can take the logic form there....