Push component should automatically recover from the server losing its uaid

[data-sync-user]

As noted in mozilla/application-services#3314 (comment) and subsequent comments, if the autopush server receives a 404 or 410 error when trying to delivery a push via one of the mobile bridges, it will completely drop the registration record for that uaid. AFAICT any attempts by the client to operate on that uaid will produce an error.

I don't think the push component currently handles this case well. In fact I don't see any codepaths that would case us to detect that self.uaid has become invalid and is being rejected by the server.

This seems like something that could be handled in verify_connection by:

• Detecting the "uaid does not exist" error.
• Registering a new uaid.
• Reporting all existing subscriptions as expired.

We should also add some handling of this case in the update method, which clients may call when in this state in an attempt to repair this push subscriptions.