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

(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
* https://github.com/civicrm/civicrm-core/pull/12362#issuecomment-400897782