Increase the default Solr indexing batch size

The default Solr batch size is currently quite low (50 documents/10k characters). This means that Solr commits currently happen quite frequently, for sufficiently large documents they could happen after every document. At the same time, Solr commits are quite expensive, meaning that when the documents to index are provided fast enough, indexing speed is dominated by Solr performing commits. Therefore, the default batch size should be increased. See also the corresponding proposal on the forum.