When running the OverdueStateApplicator logic, a lot of actions
can be triggered on different sub-systems:

* Cancellation of subscriptions
* Transition into / out of AUTO_INVOICE_OFF
* New blocking states set

These actions potentially send events on the bus that invoice will
react to, and the timing of these actions can make the resulting
invoice(s) non-deterministic.

This patch:

* Delays sending AUTO_INVOICE_OFF events until after the new blocking states are set
* Sets and unsets AUTO_INVOICE_OFF around the whole overdue logic when cancelling subscriptions

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>