Long-disconnected devices are not deleted #1544
Description
Astarte 1.2 introduced the possibility of deleting devices (see #392).
However, long-disconnected or never connected devices are stuck as "In deletion" forever.
This is due to the fact that deletion is carried on by (among other entities) a DataUpdater process in Data Updater Plant that is related to the device. When no such process exist, either because the device never connected or because it's been long since the last connection, DUP does not confirm that actual data deletion can start and therefore Realm Management does not carry it on.
We did not notice this because the Astarte dashboard uses Astarte channels in the device page. Under the hood, Channels make such a process spawn (if needed) and therefore device deletion goes smoothly.
A possible implementation path for the fix would be a RM -> DUP RPC call to signal that deletion of a given device should be started, as there is almost everything ready for it. Another option is having a background job in DUP that looks for devices currently being deleted and, if needed, spawns the necessary process(es).