fixed racing condition of the simple worker

[sio4]

The new standard (reusable) testing workflow detected racing conditions on the buffalo core, and they were all in the Simple Worker. The root cause was basically:

• There are many goroutines that use a context, and the context reading/writing caused data race.
• Functions such as PerformIn() can be called while configuring the App and they use the default context.
• Worker is started by Start(context) with a new context when App.Serve() is called after the App setup was finished.

Additionally, the Worker spec is not clear enough for each function's purpose and expected actions. We need to revisit this when we design the next version. (#2242)

• The name of Perform*() is not clear. Their actual action should be enqueuing (not to perform the job by itself) even though the Simple Worker does perform the job.
• Need to define when the job submission (calling Perform*()) is allowed.
• ...

Anyway, this PR will fix the issue for now and will make the new testing works fine. (#2265)