Made ParametersDefinerForTP thread-safe

The virtual initEvent method of ParametersDefinerForTP was const and
allowed inheriting classes to modify the state of the object. This function
has been changed to non-const and a const clone method was added. Now any
code which called initEvent must first clone the object and use their own
copy.
Remove unnecessary call that causes unloading in ROOT6
Merge pull request #3415 from wmtan/BackportAvoidUnloading

ROOT6 -- Remove unnecessary call that causes unloading in ROOT6
Merge pull request #3412 from Dr15Jones/makeParametersDefinerForTPThreadSafe

Multithreading -- Made ParametersDefinerForTP thread-safe
Merge remote branch 'origin/CMSSW_7_1_X' into CMSSW_7_0_X