Thread deletion might leave leftover messages.

Problem/Motivation

When deleting a thread, I would expect all relevant messages to be removed as well.

Currently, the \Drupal\private_message\Entity\PrivateMessageThread::delete is supposed to be loading all messages and delete them, but the underlying \Drupal\private_message\Entity\PrivateMessageThread::getMessages method, is having an edge case where it might skip a few messages.

Steps to reproduce

I will provide a test and a solution, but the main idea is:
* Create 2 users, user A, and user B.
* Create a ban from user A to user B.
* Login as user A.
* Delete the thread.

Because the user A has banned user B, they are not able to view the messages from user B because \Drupal\private_message\Entity\PrivateMessageThread::getMessages is filtering them out using the current user as a fixed source of identification.
However, the delete method also calls for this method and if there is an active session of user A, the messages found to be deleted are filtering out the banned messages.

Proposed resolution

Add an optional parameter to the ::getMessages() method to allow the delete method to call it and include banned messages.

This will retain the BC as well.

Update - 2nd issue: I have found a second issue: The \Drupal\private_message\Entity\PrivateMessageThread::delete method is responsible for deleting the relevant entities of the thread before deleting it as well as clearing the cache after that.
However, the ::delete method of the entity is not reliable for this functionality as it is mainly a wrapper around the storage ::delete method. It is not invoked by core API.

* Create a thread with some messages.
* Call $thread->delete()
* Assert that the messages are deleted. (works fine)
* Create a thread with some messages.
* Call \Drupal::entityTypeManager()->getStorage('private_message_thread')->delete([$thread]);
* Messages are not cleaned up.

Split the functionality to the designated pre/post delete static methods of the entity so that they are invoked when the entity is deleted directly from its storage class.