Make GsfElectronBaseProducer a stream module.

Static analyzer and helgrind show it is safe to convert this class
and all classes inheriting from it to be a stream based Producer.
Thread performance analysis showed this conversion is needed for
efficient threading.
Merge pull request #4087 from Dr15Jones/makeGsfElectronBaseProducerStreamModule

Make GsfElectronBaseProducer a stream module.