refine/improve the design of worker and simple worker implementation

The current design and implementation of Worker and Simple worker are somewhat ambiguous and they need to be redefined. The points of consideration could be:

• Definition of Worker and its components including functions
• How to make them more reliable (when starting, stopping, and while running)
• Workflow (both automatic parts and manual parts)

TODO

• How to check if the worker is ready to work
  • When the worker should be initialized?
  • How to check if the worker is ready?
  • and purpose of the context
• Define the purpose of Start(context) clearly
• Consider timeout and/or cancellation for background jobs
• Consider if the already-submitted jobs to be performed even if the worker canceled before they are executed

Context

• #2241 (but not directly related)
• #2198
• #2266

See Also

• #2351