Prework for #1034 - PersistentQueue

Prework for Update usage metrics: Matomo event queue, SP=5 (#1034 - closed)

This MR introduces PersistentQueueSubject's. These are ultimately meant to be used to queue offline logging events to store them and post them at a later time (see #1034 (closed)).

Sadly it was a bit more difficult than expected to add them to the usage logging system. So I separated them from the rest of the ongoing work, into a hopefully manageable package.

There are no visible changes to the app, that could be tested, but there are unit tests :)