(dev/core#174) Forms/Sessions - Clear in roughly the same ways as before

In the past, if one clears the `civicrm_cache` table (i.e.  by calling
`CRM_Core_Config::clearDBCache()` ==> `TRUNCATE TABLE civicrm_cache`), then it
has the effect of destroying any active sessions/forms.

Now, in allowing sessions to be stored elsewhere, we lose that side-effect.

If we want strictly equivalent behavior (from a business-logic perspective),
then we'd want the patch to go a bit further -- calling
Civi::cache('session')->clear() at the same time that it does
`clearDBCache()`.

This revision adds `clear()` calls to various spots discussed here:

* https://docs.google.com/spreadsheets/d/1FxuIvr2noelBvhu5eja9_ps3YUWnkmGhqijBO3gH8Po/edit?usp=sharing
* #12362 (comment)

CRM/Admin/Form/Setting.php

@@ -233,6 +233,7 @@ public function commonProcess(&$params) {
     }
 
     CRM_Core_Config::clearDBCache();
+    Civi::cache('session')->clear(); // This doesn't make a lot of sense to me, but it maintains pre-existing behavior.
     CRM_Utils_System::flushCache();
     CRM_Core_Resources::singleton()->resetCacheCode();
 

CRM/Admin/Form/Setting/UpdateConfigBackend.php

@@ -65,6 +65,7 @@ public function postProcess() {
 
       // clear all caches
       CRM_Core_Config::clearDBCache();
+      Civi::cache('session')->clear();
       CRM_Utils_System::flushCache();
 
       parent::rebuildMenu();

CRM/Core/BAO/ConfigSetting.php

@@ -260,6 +260,7 @@ public static function doSiteMove($defaultValues = array()) {
 
     // clear all caches
     CRM_Core_Config::clearDBCache();
+    Civi::cache('session')->clear();
     $moveStatus .= ts('Database cache tables cleared.') . '<br />';
 
     $resetSessionTable = CRM_Utils_Request::retrieve('resetSessionTable',

CRM/Core/Config.php

@@ -295,6 +295,7 @@ public function cleanupCaches($sessionReset = TRUE) {
 
     // clear all caches
     self::clearDBCache();
+    Civi::cache('session')->clear();
     CRM_Utils_System::flushCache();
 
     if ($sessionReset) {