This marks messages as failed if the CDN number does not match a
configured CDN number rather than falling back to the default CDN in
the event the CDN is not recognized.
You can now have a PIN without having registration lock.
Note: We still need to change the registration flow to allow non-reglock
users to enter their PIN.
This wasn't actually being used in the User-Agent header. Instead, it
was used as the value for an X-Signal-Agent header. To avoid confusion,
I'm renaming this.
