pull out heartbeat into its own class
health manager uses cf-message-bus gem [#49434789]
update nudger to allow cf-message-bus to encode json for us
Remove shadower
don't add droplets to varz
trust the cf message bus to encode/decode all json
Pull out AppInstance object to its own class
cf-message-bus properly converts all keys to symbols, so update to use the correct keys
Nudger not accessing app instance version directly
Expose AppInstance#crash_count, and using a real AppInstance in test.