Made VirtualJetProducer a stream module since fastjet is thread usable

With the modification of fastjet so that it is thread usable, different
fastjet instances do not interfere with one another, it is now safe
to make VirtualJetProducer a stream module. Both the static analyzer
and helgrind find no problems with any module inheriting from it.
Merge pull request #4081 from Dr15Jones/fastjetIsThreadUsable

Made VirtualJetProducer a stream module since fastjet is thread usable