dev/financial#84 - Simplify upgrader. Fix "wp-demo" upgrade scenario.

dev/financial#84 - Simplify upgrader. Fix "wp-demo" upgrade scenario.

Steps to reproduce
------------------

* Create a site based on `wp-demo` with Civi 5.21
* Make a DB snapshot
* Update code to 5.24
* In the web UI, run the DB upgrade
    * Note: It's important to use the web UI. The problem does not reproduce in CLI.

Before
------

The upgrader freezes on "Upgrade DB to 5.24.alpha1". The `CiviCRM.log` includes:

```
Apr 03 14:41:50  [info] Running task: Upgrade DB to 5.24.alpha1: SQL

Apr 03 14:41:55  [info] Running task: Install sequential creditnote extension

Apr 03 14:42:14  [info] $CRM_Queue_ErrorPolicy_reportError = Array
(
    [is_error] => 1
    [is_continue] => 0
    [exception] => Error 1: Uncaught Error: Class 'CRM_Volunteer_Permission' not found in /home/me/bknix/build/wpmaster/web/wp-content/plugins/civicrm/civicrm/tools/extensions/civivolunteer/volunteer.php:497
Stack trace:
0 /home/me/bknix/build/wpmaster/web/wp-content/plugins/civicrm/civicrm/CRM/Utils/Hook.php(286): volunteer_civicrm_permission(Array)
1 /home/me/bknix/build/wpmaster/web/wp-content/plugins/civicrm/civicrm/CRM/Utils/Hook/WordPress.php(139): CRM_Utils_Hook->runHooks(Array, 'civicrm_permiss...', 1, Array, NULL, NULL, NULL, NULL, NULL)
2 /home/me/bknix/build/wpmaster/web/wp-content/plugins/civicrm/civicrm/Civi/Core/CiviEventDispatcher.php(86): CRM_Utils_Hook_WordPress->invokeViaUF(1, Array, NULL, NULL, NULL, NULL, NULL, 'civicrm_permiss...')
3 /home/me/bknix/build/wpmaster/web/wp-content/plugins/civicrm/civicrm/vendor/symfony/event-dispatcher/EventDispatcher.php(184): Civi\Core\CiviEventDispatcher::delegateToUF(Object(Civi\Core\Event\GenericHookEvent), 'hook_civicrm_p
    [last_task_title] => Install sequential creditnote extension
)
```

After
-----

The upgrade completes. After installation, the `sequentialcreditnotes` extension is active.