Make GsfElectronCoreBaseProducer a stream module

The static analyzer and helgrind report that all classes that inherit
from GsfElectronCoreBaseProducer are safe to be stream modules.
Merge pull request #3939 from Dr15Jones/makeGsfElectronCoreBaseProducerStreamModule

Multithreading -- Make GsfElectronCoreBaseProducer a stream module
Merge remote branch 'origin/CMSSW_7_1_X' into CMSSW_7_0_X