New class: SOLR commit document

* VuFindSearch/Backend/Solr/Document/CommitDocument.php: New
  class. SOLR commit document.
New class: SOLR commit document

* VuFindSearch/Backend/Solr/Document/CommitDocument.php: New
  class. SOLR commit document.
Avoid fatal Demo driver error when index is empty.
Add some default parameter values.
Avoid notice.
Very simple serializable record object.
Added more \VuFind\Solr\Writer functionality.
Updated index_reserves.php to use \VuFind\Solr\Writer.
Added aliases to access backends by core names.
(For legacy compatibility).
Updated optimize to use \VuFind\Solr\Writer service.