CRM-16373 - Migrate config_backend to civicrm_setting

CRM-16373 - Migrate config_backend to civicrm_setting

There are two steps at which we convert config_backend:

 * In loading the upgrade environment, we may need access to old settings.
   So SettingsBag supports reading config_backend for the active domain.
 * As a specific upgrade step (for 4.7.alpha1), we migrate all values
   for all domains (unless there's already a setting).