[CX] utilize a lightweight scaffolding (sort of) web server in 'cbas' while driver is down

Currently when a driver is down, all customer-facing endpoints return connection refused. Given that the 'cbas' wrapper wholly owns the driver lifecycle, it is feasible that it could bind to :8095 and provide a better response (e.g. 503 - Service Unavailable) in this event.

For extra credit, we could also consider mirroring or relocating the config servlet here, to provide a way to recover from bad jvmArgs without having to do metakv hacks or remove the node from the cluster.