Fix thread safety issue with PosteriorWeightsCalculator

Changed the code to call the thread safe version of KfComponentsHolder::setup
which avoids uses of a static.
Merge pull request #3600 from Dr15Jones/fixThreadSafetyInPosteriorWeightsCalculator

Reco -- Fix thread safety issue with PosteriorWeightsCalculator
Merge remote branch 'origin/CMSSW_7_1_X' into CMSSW_7_0_X