Use handler map for query defaults, appends, invariants

* VuFindSearch/Backend/Solr/HandlerMap.php (__construct): Make map an
  optional argument.

* VuFindSearch/Backend/Solr/Connector.php (getQueryInvariants)
  (getQueryDefaults, getQueryAppends, setQueryInvariants)
  (setQueryDefaults, setQueryAppends, addQueryInvariants)
  (addQueryDefaults, addQueryAppends, $invariants, $defaults)
  ($appends): Removed.
  (getMap): New function. Return handler map.
  (retrieve, similar, search, terms): Use handler map to prepare
  params and select handler.
  (select, prepare): Removed.