The last to_device message comes down initial sync

[erikjohnston]

@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 🤷