Add separate process that listens for subscription changes

Uses 0mq to communicate between registrar and subscriber. Uses
a common file stored in a known location to communicate on which
host and port the registrar is running. Clients should be able
to do operations on keys only if they already know the key.

FIXME - Autogenerate the key, rather than let the client provide
it. Our bot authors will just provide dummy keys, never secure
ones

FIXME - Remove the registrar address file when the registrar
exits. This prevents other people from trying to 'fake' the
registrar by crashing the process and binding to the older
port

Change-Id: I203881dfc15a59cb629d047c33c14f5b60390627