The preview hub throws an exception during DB upgrade

[kjac]

Prerequisites

• I have added steps to test this contribution in the description below

Description

The preview hub sometimes throws an exception when upgrading a DB:

An unhandled exception has occurred while executing the request.
Umbraco.Cms.Core.Exceptions.BootFailedException: Boot failed: Umbraco cannot run. See Umbraco's log file for more details.

-> Umbraco.Cms.Core.Exceptions.BootFailedException: An error occurred while running the unattended upgrade.
The database configuration failed with the following message: One or more errors occurred. (No key is set for payload with id 0)
 Please check log file for additional information (can be found in 'LoggingSettings.Directory')

I can't entirely put my finger on when and why, but it has to do with content cache refreshing at the end of the upgrade process:

This PR adds a little extra logic to the preview hub, so it strictly only handles "refresh node" notifications.

Testing this PR

To test that the upgrade works, get in touch with @kjac for a test DB which will trigger the cache refresh during upgrade.

Also test that the preview hub actually triggers updates when saving content. It's likely easiest to verify this using a debugger.

[Zeegaan]

Looks good, tests good 🚀