INT-3485 Remove final Modifier AbstractEndpoint.stop(Runnable callback)

JIRA: https://jira.spring.io/browse/INT-3485

I have signed and agree to the terms of the SpringSource Individual Contributor License Agreement.
@kapr , thank you for contribution!

Here you should really invoke that abstract `doStop()`, not `stop()` with one more `lock` acquiring.
Further:
* The test-case on the matter is required.
* Add your name to author list
* Update `copyright`
* Minor: do not use `this.` for methods
* no blank lines in the method JavaDocs

For the future: please, correctly format your first commit message:
* headline <= 50 symbols
* JIRA link over a blank line after header
* Some minor description