bugfix: protect the job function's waitgroup with a mutex to avoid race conditions

[JohnRoesler]

What does this do?

a possible race condition could occur with the job function's sync.WaitGroup being accessed from multiple goroutines

• the initial creation (in the scheduler goroutine)
• the adding within the executor goroutine)
• the waiting within the executor stop goroutine (which could be different from the scheduler stop if stop is called from another goroutine by the user)

Which issue(s) does this PR fix/relate to?

resolves #507

List any changes that modify/break current functionality

Have you included tests for your changes?

Did you document any new/modified functionality?

• Updated example_test.go
• Updated README.md

Notes