When changing credentials old cache prevents connection

The old persistency of a device might prevent device connection when the credential secret has been changed.

To replicate:
- Connect a device to MQTT using a credential secret.
- Disconnect and destroy the device.
- While disconnected, wipe the device credentials in the Astarte instance. Then perform the manual registration once more obtaining a new credential secret.
- Do not modify the device persistency folder. Attempt to reconnect the device using the new credential secret.

The device will not return an error but cannot connect to Astarte.

Start by reproducing the MQTT connection sequence with the existing persistency folder and a newly issued credential secret. Trace how persisted credentials are loaded and used after device destruction; done means the device reconnects to Astarte with the new secret without requiring manual changes to the persistency folder.

Written by the indexing model from the issue text.