lump-commit of additions from  track cluster splitting ported from 5X to 71X and SLC6 #2961. Porting of cluster splitting in 71X release. Modified for the new tracking.
Properly handle a default constructed edm::RefToBase

Several methods of edm::RefToBase assumed holder_ was properly
set but that is not the case for the default constructor. These methods
are now properly protected against a null pointer.
Merge pull request #3352 from slava77/for-71X-tkClustSplitting-from-taroni-71X_SLC6

Reco -- Track cluster splitting ported from 5X to 71X and SLC6 (recommit #2961)
Merge pull request #3353 from Dr15Jones/fixRefToBaseDefaultConstructed

Core -- Properly handle a default constructed edm::RefToBase
Merge remote branch 'origin/CMSSW_7_1_X' into CMSSW_7_0_X