Add probe endpoint for startup

[weeco]

With Kubernetes 1.16 becoming more common I think it's wise to utilize the newly introduced startupProbe. To date Kowl exposes just one endpoint for probes, the /health endpoint. In order to make sure that Kowl doesn't receive traffic before it has connected to all brokers in the Kafka cluster we use the /health endpoint for the readiness probe.

Kowl is capable of handling transient connection issues (to Kafka) and therefore shouldn't be killed by any container orchestrator unless it's unrespsovie. Thus I propose to introduce another endpoint for probes:

• /health - Returns static ok from the webserver. This is used to check whether the HTTP server is still alive
• /startup - Returns ok if connectivity to (all) Kafka Brokers is given. This probe can be used for readiness probes but if possible a startup probe should be used in favour of using a readiness probe at all.