Inject backend identifier via setter, not constructor

* VuFindSearch/Backend/Solr/Backend.php (__construct): Use object hash
as default identifier.
* VuFindSearch/Backend/Solr/Backend.php (__construct)
(setIdentifier): Inject backend identifier via setter, not
constructor.
* VuFind/Search/BackendManager.php (onResolve): Inject backend
identifier.
* VuFindSearch/Backend/BackendInterface.php (setIdentifier): Make
setIdentifier() part of the backend interface definition.
* VuFind/Search/Factory/SolrReservesBackendFactory.php (__construct):
  Dto.
* VuFind/Search/Factory/SolrDefaultBackendFactory.php (__construct):
  Dto.
* VuFind/Search/Factory/AbstractSolrBackendFactory.php
($serviceName): Removed. Name injected by BackendManager.
* VuFind/Search/BackendManager.php (onResolve): Inject backend
  identifier.