Implement SOLR terms operation

* VuFindSearch/Backend/Solr/Response/Json/Terms.php: New
  class. SOLR term component output.
* VuFindSearch/Backend/Solr/Backend.php (terms): New
  function. Return list of terms.
* VuFindSearch/Backend/Solr/Connector.php (getTerms): Removed.
  (query): Declare public. To be called by Backend.
Log SOLR query time

* VuFindSearch/Backend/Solr/Backend.php (deserialize): Log SOLR query
  time.