Without this, old senders will throw a messy 404 traceback when talking
to a modern server.
Unfortunately 0.4.0 receivers don't make API calls in the right order,
so they throw a 404 before seeing our "you need to upgrade" message.
I was really confused about the Server-Sent Events syntax. This new one
is compatible with actual web browsers and the spec:
http://www.w3.org/TR/eventsource/
This requires a DB delete/recreate when upgrading. It changes the server
protocol, and app IDs, so clients cannot interoperate with each other
across this change, nor with the server. Flag day for everyone!
Now apps do not share channel IDs, so a lot of usage of app1 will not
cause the wormhole codes for app2 to get longer.
