Store resolved backend in event parameters.

* VuFindSearch/Service.php (search, retrieve, similar): Store resolved
  backend in event parameters.

Resolved backend is stored as `backend_instance'.
Inject response writer in Solr Backend

* VuFindSearch/Backend/Solr/Backend.php (injectResponseWriter):
  New method. Inject response writer.
  (alphabeticBrowse, terms, similar, retrieve, search): Use new
  method.
Only set fields as query defaults, wt and json.nl set in Solr Backend.

* VuFind/Search/Factory/AbstractSolrBackendFactory.php
  (createConnector): Only set fields as query defaults, wt and
  json.nl set in Solr Backend.