Replacing PR#5954: Threadsafe faster hcal25ns reco method2

 A new PR. This is the SAME as the latest update of the PR#5954 and meant to replace that (with a fresh pull request...)

(ref: PR#5954: https://github.com/cms-sw/cmssw/pull/5954)
I see that you have removed the copyright notice.
After I learned that this is essentially a copy of the Minuit2Minimizer, the copyright note should be put back and some notes should be added about modifications made in this code.
Please also write a message on roottalk to investigate if there are subtle reasons to not support changes in the fit strategy.