KubeService liveness/readiness probes

[jefflill]

BREAKING: We need to enhance the KubeService to optionally write some kind of readiness file to disk that a Kubernetes probe script could examine to determine the worker health. In theory, this will have to work on all platforms (Windows, Linux, and OS/X) as well as for unit testing.

Service developers can enable this by passing the the fully qualified path to the status file as the new optional statusFilePath parameter to the KubeService constructor. By default, this will be NULL, indicating that no file should be created.

You'll need to code your health probe to examine this file. If the file doesn't exist, your probe should assume that the service hasn't been started yet. If the status file does exist, it will have one of the following text values with no line ending characters:

not-started
starting
running
unhealthy
terminated

Note that there's one related breaking change: KubeService.SetRunning() has is now async and has been renamed to SetRunningAsync(). You'll need to update your services,

[jefflill]

Implemented for: v2.0.0