Improve kill task logic

[pditommaso]

This PR improves the task kill logic as below:

1. implement the flag check in the kill method (instead of a new one) for a) API backward compatibility, and b) make sure it's invoked by the already existing component
2. use an AtomicBool to make the handling of a possible race condition more robust
3. rename existing kill methods to killTask and declare protected to highlight it should not be invoked directly
4. do not mark kill as final (even if it would be better) to prevent breaking asking plugins