Do not prune workers from queues that we don't listen to.

If the worker we are trying to prune does not belong to the queues we are listening to, we should not touch it. 

Attempt to prune a worker from different queues may result in an unknown class exception, since that worker could easily be even written in different language.
Merge pull request #1000 from dsabanin/dont-prune-foreign-workers

Do not prune workers from queues that we don't listen to.