`device_federation_inbox` table is never emptied

[richvdh]

device_federation_inbox is used to keep a record of the to-device messages we have received and processed from remote servers, to avoid double-processing.

However, it is never cleared out and can get quite large.

(It does not actually store the content of the messages; that goes into device_inbox.)

[richvdh]

Since this table only exists to give request idempotence, it would be relatively easy to write a timer task to clear out rows more than, say, a week old; we'd probably need to add an index on the timestamp column though.