Use factory function to create records of a SOLR record collection

* VuFindSearch/Backend/Solr/Response/Json/RecordCollectionFactory.php
(__construct, factory): Use factory function to create collection
records instead of classname.
* VuFind/Search/Factory/SolrDefaultBackendFactory.php
(createBackend): New function. Set RecordCollectionFactory with
factory function for SOLR records.