Make $conf['cache'] configurable via disable_drupal_page_cache INI variable.
Nginx: Send Boost compatible Cache-Control headers also with Speed Booster active.

This is to mimic Drupal core behaviour when full-page cache is disabled, even if it is not really disabled via disable_drupal_page_cache INI variable. Note that Speed Booster continues to ignore Cache-Control headers sent by Drupal backend, as before, to force its own TTL set via speed_booster_anon_cache_ttl INI variable or in the custom local.settings.php code.