/healthcheck URI just returns a 200/ok for cloud service monitoring

[mberkowski]

This adds a URI under the prefix path /healthcheck that merely returns 200/ok without invoking the URI factory. It is to be used by monitoring services that are only concerned with whether Node is responding to inputs but need not form a redirect.

e.g.

• http://localhost:1337/healthcheck if uriPathPrefix == '/'
• http://localhost:1337/janus/healthcheck if uriPathPrefix == '/janus'

It might be more ideal for this to be an implementation detail in janus-deploy instead, but the router isn't expose there to add additional routes at this time.