Call CRM_Core_Config::clearDBCache(); in hook_flush_caches() for Drupal

I've already created a module to handle this https://drupal.org/project/civicrm_clear_all_caches. It works as expected. @lobo asked me to submit this as PR to civicrm-drupal, so I'm doing that. I do have some concerns about adding this to civicrm.module because you would no longer have the option of clearing just Drupal's cache. You could clear just CiviCRM's cache using civicrm/admin/setting/updateConfigBackend?reset=1, but admin/config/development/performance would always clear both Drupal and CiviCRM. Leaving this as an optional module would allows users to decide which caches they clear when using admin/config/development/performance. This could also be controlled w/ a configuration option in CiviCRM. That's probably the best solution, but I don't know where that option should live. I'll submit the initial PR and wait for feedback.