AGENT_REMOVED event not sent when agents marked GONE

The master currently does not send subscribers the AGENT_REMOVED event when agents are marked GONE, but it should.

Since the _removeSlave method is used to handle both the UNREACHABLE and GONE cases, we could update it to conditionally send this event. However, it's worth noting that the _removeSlave/_removeSlave logic is messy and unintuitive and in need of refactoring - I suspect we can turn these into a single method which handles all cases with the help of an auxiliary function or two.