The setting for making an index read-only is broken

The setting "Index write access" at /admin/config/search/apachesolr, which allows you to prevent Drupal from writing to the index, is broken after the move to per-server settings.

The apachesolr_read_only variable is being stored in the main Drupal variable table (via system_settings_form). The Apachesolr module expects this variable to be stored in its own environment-specific variables table. This means the variable is never properly loaded and always falls back to APACHESOLR_READ_WRITE, leaving the index open to writes. I'm setting the priority to 'major' since this allows you to inadvertently kill an index while testing.

The issue #1020780: Store variable settings per server for better flexibility involves changes that lead to this problem.