Update example
Insert empty Array even if releases not exist
clean task now see old releases
todo
package_status: Don't see master nodes
PackageStatus#to_hash
Status for preparation, switching
memoize
Don't include 'done' agents in working/queued list

- task might left as working due to event processing order
  - if proceeded finish event first
Should be unique
Remove useless codes
enqueue clean task on finish
actions#switch: Accept :no_release
Add switch task, release:remove handler
api: support package status
api: support switch task request
cli/client: support package-status
cli/client: support switch request