Lazy-load field facets, expose query facets

* VuFindSearch/Backend/Solr/Response/Json/Facets.php
  (getFieldFacets): Lazy-load field facet information.
  (getQueryFacets): New function. Return lazy-loaded query facet information.
  ($facets): New property. SOLR facet information.
  ($queries): New property. Query facets.
Define WriteableBackendInterface

* VuFindSearch/Backend/Feature/WritableBackendInterface.php (delete):
  New interface.
* VuFindSearch/Backend/BackendInterface.php (similar): Moved to
  Feature/WritableBackendInterface.
Define methods to fullfill WritableBackendInterface

* VuFindSearch/Backend/Solr/Backend.php (update, deleteAll)
  (delete): Define methods to fullfill WritableBackendInterface.