Fix #835 and #837

[apocelipes]

What does this do?

Fixes #835.
Fixes #837.

For 837: e.stopCh now has only one receiver, and if executorCtx.Done() does not block, it means that the executor has already received the stop signal and called the stop method, so there is no need to do additional operations, just return from the sub-goroutine.
I also use sync.Once to prevent the stop method from being called multiple times before the next call to the start method. This may not be necessary, but it maximizes concurrency security, and it doesn't do any noticeable harm.

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

#835
#837

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

The two issues have some relevance, so I put the fixes in the same PR. Please don't sqush the commits.

[JohnRoesler]

Thanks for digging into these. Concurrency is hard!