added input/status checks, testcases, and comments for road-to-v1

[sio4]

Improved the simple worker:

• Added input/status checks for
  • Register(), if the name and handler are defined correctly
  • Perform() and PerformIn(), if the worker's context was already canceled to prevent job submission
• Added wait group count for delayed jobs (PerformIn)
• Added locking in Stop() to prevent job submission when stopping
• Added comments for the road-to-v1 (refine/improve the design of worker and simple worker implementation #2242)

(This PR was started for #2241)

[sio4]

This PR was started for #2241 but the root cause of the issue was not here. However, this PR will make Simple Worker more reliable.

[paganotoni]

Thanks for this one @sio4

[sio4]

Thanks!

[fasmat]

Thanks @sio4, looks good to me!