replace ttl-based crashing with murder

Nodes now keep their registry in memory instead of etcd; there is now only one
key to store per app instance. Yay!

Also instead of using a TTL for an app crashing, on startup they are marked
for death, and ther owning node brutally kills them after 5-10 seconds.

This allows the heartbeating demonstration to be done at the same time as
crashing instances. Removed Procfile.heartbeats.