The last `to_device` message comes down initial sync

This issue has been migrated from [#9767](https://github.com/matrix-org/synapse/issues/9767).

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@manuroe discovered this while writing integration tests:

> so, FTR, synapse seems to always send the last to_device event for an initial sync even after several incremental syncs after I receive this to_device
> if there is a new to_device event in those intermediate incremental syncs, the initial sync will not return my first to_device event but only the second one
I have my initial data for my test. I am happy now but I just wanted to share it here
I tested it against Synapse/develop

It's possible that this is happening because the global `to_device` token isn't incremented as no other to device messages are sent, but :shrug: