Separate app support

[palfrey]

Right now, notifiers get all events. This is fine if there's only one type of app talking to Potboiler, and all the instances of that app talking to Potboiler nodes want to interact. OTOH, if you want multiple apps, or multiple versions (in the sense of use, not version number) of an app to be on the same Potboiler network, things get a bit harder.

This ticket covers adding an app id to all messages, notifiers registering for event saying what their id is, and only messages with that id get sent to a notifier. Possibly also a default "send everything" option as well as designating a default app id for KV just so for the often "every KV wants to talk to every other" scenario is easy.