Expose function to set, get, and add handler parameters

* VuFindSearch/Backend/Solr/HandlerMap.php (getParameters)
  (setParameters): Declare public, rename parameter $key to $type.
  (addParameter): New function. Add parameters.
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.
Use handler map to create SOLR connector

* VuFind/Search/Factory/AbstractSolrBackendFactory.php
  (createConnector): Use handler map.
Factor out d/a/i application in AbstractHandlerMap

* VuFindSearch/Backend/AbstractHandlerMap.php (apply): New
  function. Apply defaults, appends, invariants.
  (prepare): Factor out d/a/i application, undeclare final.
Merge branch 'master' of ssh://vufind.git.sourceforge.net/gitroot/vufind/vufind
Correctly set fallback handler

* VuFindSearch/Backend/Solr/HandlerMap.php (setHandlerMap):
  Correctly set fallback handler.
Rename Feature/MoreLikeThis to Feature/SimilarInterface
Create test Connector with dummy handler map

* ../tests/unit-tests/src/Search/Solr/MultiIndexListenerTest.php
  (setup): Create Connector with dummy handler map.
Force ChangeTracker to always work in UTC time.
Resolves VUFIND-659.