For DefaultConnection, default to gevent only if actually patched.
For DefaultConnection, default to gevent only if actually patched.

PYTHON-244
Fixes an issue where gevent is chosen based on module loaded, not monkey
patching.
In some debuggers, (PyCharm, for example), this would cause connecting
to timeout because gevent reactor is used without monkey patching.

mobify/python-driver,markflorisson/python-driver,kishkaru/python-driver,kracekumar/python-driver,tempbottle/python-driver,mike-tr-adamson/python-driver,vipjml/python-driver,jfelectron/python-driver,tempbottle/python-driver,stef1927/python-driver,beobal/python-driver,thobbs/python-driver,coldeasy/python-driver,bbirand/python-driver,mobify/python-driver,jregovic/python-driver,mambocab/python-driver,mike-tr-adamson/python-driver,kracekumar/python-driver,thobbs/python-driver,stef1927/python-driver,jregovic/python-driver,mambocab/python-driver,datastax/python-driver,coldeasy/python-driver,thelastpickle/python-driver,vipjml/python-driver,markflorisson/python-driver,yi719/python-driver,bbirand/python-driver,kishkaru/python-driver,jfelectron/python-driver,yi719/python-driver,datastax/python-driver,thelastpickle/python-driver,beobal/python-driver