Make it possible to configure the batch size in the Solr index synchronization on startup

The index synchronization on startup currently uses a batch size of 100 documents. On big wikis, as noted in XWIKI-18817, this leads to a huge load on the database as the query performance is poor. While waiting for a proper solution, a huge improvement would already be if it was possible to increase the batch size such that less of these expensive queries are performed when there is enough memory. Additionally, the default should be increased to 1k to bring at least a 10x improvement.