This diff introduces handling of the newNotifyToken in Get stream request.
When the device has an invalid or expired notification token we are requesting a new token from the client (ref to D5207).
The client gets back with the new token in Get stream request when starts listening for new messages (getting messages) from the Tunnelbroker.
This is a part of the stack.
This approach was used keeping in mind the transition to the bidirectional stream soon.
I thought putting an invariant here was a reasonable trade-off vs. refactoring the types in the fullStateSyncActionType payload (specifically StateSyncFullActionPayload which is derived from ClientFullStateSync which is derived from BaseFullStateSync) which are used across clients and keyserver.
Open to re-exploring if that would be preferred.