Fix thread safety issue with PosteriorWeightsCalculator

[Dr15Jones]

Changed the code to call the thread safe version of KfComponentsHolder::setup
which avoids uses of a static.

[cmsbuild]

A new Pull Request was created by @Dr15Jones (Chris Jones) for CMSSW_7_1_X.

Fix thread safety issue with PosteriorWeightsCalculator

It involves the following packages:

TrackingTools/GsfTracking

@nclopezo, @cmsbuild, @anton-a, @thspeer, @slava77, @Degano can you please review it and eventually sign? Thanks.
@gpetruc, @cerati, @GiacomoSguazzoni, @rovere, @venturia this is something you requested to watch as well.
You can sign-off by replying to this message having '+1' in the first line of your reply.
You can reject by replying to this message having '-1' in the first line of your reply.
@nclopezo, @ktf you are the release manager for this.
You can merge this pull request by typing 'merge' in the first line of your comment.

[nclopezo]

I am having troubles again with git-cms-merge-topic.

fatal: Couldn't find remote ref refs/pull/3600/head

The tests are running, I will post the results by hand.

[nclopezo]

+1
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-Dr15Jones:fixThreadSafetyInPosteriorWeightsCalculator/1237/summary.html

[slava77]

+1

for #3600 f8b69a2

jenkins ok

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_1_X IBs unless changes (tests are also fine). @nclopezo, @ktf can you please take care of it?