Fix to-device being dropped in limited sync in SQLite. (#11966)

If ther are more than 100 to-device messages pending for a device
`/sync` will only return the first 100, however the next batch token was
incorrectly calculated and so all other pending messages would be
dropped.

This is due to `txn.rowcount` only returning the number of rows that
*changed*, rather than the number *selected* in SQLite.