controller: Track jobs possibly started before the controller

Both controller and postgres jobs can be seen with `flynn ps`:

```
vagrant@flynn:~/go/src/github.com/flynn/flynn$ flynn apps
ID                                NAME
db49ae4840d2448f9d33746c535e371d  gitreceive
cd3a804c55ac4c69baf63d05c933d9ae  router
6c00263e0974445b912d74e5236477e2  blobstore
0bc8a5c4232b4779bcf18e47a8376294  postgres
26513bdcdd534bdf8d13b46c7b57b64f  controller
vagrant@flynn:~/go/src/github.com/flynn/flynn$ flynn -a controller ps                                                                                            
ID                                      TYPE
flynn-9f22dc5501fa46388a4ee879530c8ee7  web
flynn-61c6dda8c1a643a6b98dd986995fafc2  scheduler
vagrant@flynn:~/go/src/github.com/flynn/flynn$ flynn -a postgres ps                                                                                              
ID                                      TYPE
flynn-8263af3e2d544a3a80c6bbe537b0cf1d  web
flynn-5578e8d642ec4354ac593f9044054023  postgres
```

Closes #37