During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
We were incorrectly checking if the *local* token had been advanced, rather than the token for the remote instance.
In practice, I don't think this has caused any bugs due to where we use `wait_for_stream_position`, as critically we don't use it on instances that also write to the given streams (and so the local token will lag behind all remote tokens).
