KfComponentsHolder cleanup

Cleanup of KfComponentsHolder.
It became a bit more urgent due to changes introduced by thread-safety.
Restore the timing performance of the Chi2MeasurementEstimator (still pretty deceiving though).

No regression expected, no regression observed
the function is used and the client code has been fixed (se above)
for what concern code not in CMSSW 
depending how it is coded it may not compile and the compiler error will indicate how to fix it.
We are bounded to fix code in CMSSW. private code is responsibility of the respective maintainers