Centralise buildengine build update logic

[matt2e]

Each time a build starts/fails/succeeds we have to:

• log the update
• tell e.listener
• tell the CLI status manager

It's easy to miss doing one of these in each case, or let the logs drift away from being consistent.

• Could logging be moved to the e.listener wrapper functions rather than being spread out?
• Can we support pubsub instead of a single listener?
• Can we connect the build engine and the CLI status manager via a listener?

[stuartwdouglas]

I think it might also be worth trying to remove the 'round' based processing, and move to a direct dependency model instead. The round based approach means that some deployments can end up waiting for unrelated deployments in the earlier round.