Prototype SOLR update document

* VuFindSearch/Backend/Solr/Record/SerializableRecordInterface.php:
  New interface. Serializable SOLR record.
* VuFindSearch/Backend/Solr/Document/UpdateDocument.php: New
  class. SOLR update document.
New function: Save a SOLR update document

* VuFindSearch/Backend/Solr/Connector.php (save): New function. Save a
  SOLR update document.
Fix docstring Solr/Connector::setAdapter()

* VuFindSearch/Backend/Solr/Connector.php (setAdapter): Fix docstring.