New function: Return parameters as array

* VuFindSearch/ParamBag.php (getArrayCopy): New function. Return
  parameters as array.
Return prepared params

* VuFindSearch/Backend/Solr/Connector.php (prepare): Return
  prepared params.
  (select, alphabeticBrowse, getTerms): Use return value.
Implement SOLR Connector query appends and defaults

* VuFindSearch/Backend/Solr/Connector.php (__construct):
  Initialize query invariants, defaults, and appends.
  (getQueryDefaults, getQueryAppends, setQueryDefaults)
  (setQueryAppends, addQueryDefault, addQueryAppend): Set, get, add
  to query defaults and appends.
(prepare): Compose final parameters.
* VuFind/Search/Factory/AbstractSolrBackendFactory.php
  (createConnector): Add parameters as query appends, initialize
  defaults.
Include request duration in log output

* VuFindSearch/Backend/Solr/Connector.php (sendRequest): Include
  request duration in log output.
Retrieve defaults and appends array

* VuFindSearch/Backend/Solr/Connector.php (prepare): Retrieve
  defaults and appends array.