Candidate Optimized 

The modification in this PR were discussed in many reco and edm meetings and HN threads.
Candidate Hierarchy has been made thread safe and optimized.
Due to a missing versioning till ~7_1_0_pre3,  schema evolution requires to read files generated with 7_1_0
including GenSim.

I have tested with new relvals and now everything works.

The branch seems to be in a messy state due to many merge, still the list of file changed is correct