Set Vx3DHLTAnalyzer to thread_unsafe::DQMEDAnalyzer

The Vx3DHLTAnalyzer uses a global std::vector to hold all Vertices
it has seen. Filling this on different threads will lead to crashes
of cmsRun. Therefore changed it to inherit from thread_unsafe::DQMEDAnalyzer.
Merge pull request #4429 from Dr15Jones/setVx3DHLTAnalyzerToThreadUnsafe

Set Vx3DHLTAnalyzer to thread_unsafe::DQMEDAnalyzer