remove eager callback execution in timer
remove eager callback execution in timer

With our fix for PYTHON-755, you can now start timers with a negative
timeout in cluster.Cluster._start_timer. When this happens,
Timer.callback can be called twice, once in Timer.__init__ and once in
Timer.finish. The first is just an optimization, so we remove it here.

datastax/python-driver,mambocab/python-driver,thelastpickle/python-driver,thelastpickle/python-driver,mambocab/python-driver,datastax/python-driver