Support switching the account that runs the operation

The current implementation on the MRs does the account switching in an event subscriber. This, however, causes a problem in that
switching back will not happen if there's an error in the following cases:

- If an operation is cancelled by a pre-initiate subscriber.
- If there's a fatal error in another pre-initiate subscriber.
- If there's a fatal error in another post-terminate subscriber.

Here's example code.

$cancel = $this->preInitiate(
      Events::REMOTE_LIST_PRE_INITIATE,
      'import_list',
      $context,
      $sync
    );
    if ($cancel) {
      return;
    }

    try {
      $this->doImportRemoteList($sync, $filters, $options, $context);
    }
    finally {
      // Notify subscribers that the operation has terminated.
      $this->postTerminate(
        Events::REMOTE_LIST_POST_TERMINATE,
        'import_list',
        $context,
        $sync
      );
    }

We therefore need to move account switching from an event subscriber to a service that is used by the import/export manager.

- Move the logic to a service called AccountSwitcher
- In all relevant places in both the import and the export manager, switch accounts before pre-iniate events.
- Wrap pre-initiate events in try/catch, switch back accounts, re-throw the error.
- Switch back accounts if the operation is cancelled.
- Wrap post-terminate events in try/catch, switch back accounts, re-throw the error.

Now, here's the other complexity. An operation can have dependent operations i.e. it can call or be called within the exeuction of another operation. That's actually a pretty common use case.

Imagine the following:

- Operation A switches from user 0 to user 1.
- Operation A runs.
- Before terminating, operation A triggers the execution of operation B.
- Operation B switches from user 1 to user 12.
- Operation B runs.
- Operation B terminates, at which point needs to switch back from user 12 to user 1.
- Operation A terminates, at which point needs to switch back from user 1 to user 0.

We therefore need to track in a variable whether the account switched and then use that result to switch back.