device_inbox never gets emptied

[matrixbot]

To-device messages remain in device _inbox until the message is either successfully delivered (as indicated by the client sending a sync token which indicates that it must have received the message), or the recipient device is deleted.

It's therefore possible to accumulate a significant number of to-device messages which are unlikely ever to be delivered, because the recipient device no longer exists (but has not been explicitly logged out). (Related: #8263).

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This issue has been migrated from #3599.

[richvdh]

For links: the stream tables (including device_federation_outbox) are covered by #5888.

[richvdh]

Have opened a separate issue to track device_federation_inbox: #17370